I've encountered a weird issue with Morrowind in which attacking NPCs get "stuck" and stand there motionless while I kill them. Sometimes the attack animation runs for a few seconds and then freezes. Sometimes they're stuck from the beginning, and the only way I know they're attacking is by trying to talk to them at which point the game tells me they're attacking.

I'm using the GOG game, not OpenMW, modded with Mod Organizer 2, with a mod list loosely derived from Morrowind 2020: Thastus Edition. Most of the mods I've installed are related to graphics, user interface/HUD tweaks, etc., very little that I would expect to affect game mechanics or AI. So I'm stumped as to where to start looking for a solution to this problem.

Any help appreciated.

Try disabling your mods, loading your saved game, and seeing if the problem persists.

If that fixes the problem, you may need to do some bisection to figure out what mod is the issue. You can try enabling mods one at a time, or if it's a lot, enable about half of them to try and narrow down the problem mod faster (though there's the possibility of the issue being a combination of some mods).

Thanks. Yeah, normally that's how I'd proceed, but frustratingly that approach doesn't work well here because the problem's intermittent. I encountered a good example yesterday when I just happened to encounter the problem shortly after saving the game. A few moments after saving, I picked a locked door and a nearby NPC yelled at me and then just stood there stuck. So I loaded the recent save and picked the same door. This time the NPC yelled at me and then attacked normally. I've tried the same save file with a couple other MO2 profiles I have with different mod lists (including one with no mods) hoping I'd get a repeatable "stuck NPC" situation. Sadly, the NPC behaves properly in all cases which of course proves nothing. :-(

Without a repeatable test case, it's nearly impossible to identify the mod (if any) that's at fault with the "binary search" approach.

Maybe there's a console command I can use to get info about a stuck NPC if/when the problem recurs, or something along those lines that would yield a clue?

Repeatable or not - it all depends on how much time and effort you're willing to invest. The first thing I'd probably do is, following dtgreene's advice, to play without mods, to find out if it's a problem of the base game or of a mod. If the base game is ok, you can make up your mind, whether or not you want to dig deeper.
Otherwise I'd probably try a clean install of the base game (having backed up all save games and completely uninstalled your current installation, including all registry entries), to rule out the possibility that there is something wrong with the registry entries or that a mod has changed base game files. Another thought is to run Morrowind outside of Galaxy (if you use Galaxy now). And I'm not sure if a corrupt save game could produce such an effect or not

Perhaps. I appreciate that perspective.

I'm hoping maybe somebody could offer a suggestion for a more directed diagnostic approach. For example, what's more likely to affect NPC behavior: records in an ESP or MWSE script? Any hints about how NPC behavior (esp. attack behavior) is determined or controlled? Perhaps somebody's read a post somewhere about similar behavior. That kind of thing.

(And FWIW I've already tried the "first-resort" things like reinstalling from scratch, comparing my current game folder to a copy of a virgin installation, etc., and I don't use Galaxy.)

I still think it would be most important for you to find out if the problem occurs when you play the base game without any mods, too. Depending on the result, your future investigation would take place in two very different worlds and you could do it in a much more focussed manner.
For the "base game route", I can't offer you any help - I've played Morowind for 100's of hours and never encountered a problem simillar to yours (if I remember correctly there is an exploit which makes fighting monsters near doors easier). If you want to investigate the mod side further, you could search the internet for "morrowind mod conflicts" - for example, uesp.net contains some information. It may or may not give you a clue, but it might be worth trying.
Good luck!

I appreciate you taking time to respond.

Modded vs. unmodded is pretty simple; just switch MO2 profiles. As a test, I went through Hlormaren (lots of hostile NPCs) twice, once modded and once unmodded. When I went through there recently with my modded game, two of the NPCs exhibited the "stuck" behavior.

With an unmodded game, all NPCs attacked immediately and vigorously. Then I switched to my modded profile, loaded an earlier save, and coc'd my way over there again. All NPCs (wait for it... :-) attacked immediately and vigorously. That is, the game's behavior is inconsistent even at a given location.

Because of that fact, not encountering the problem doesn't really tell me anything. To put it another way, it's impossible to know how long I'd need to play with a different set of mods to "prove" that the problem's not going to occur.

So far the only hint I have is that when the problem does occur, loading an earlier save seems to clear whatever condition is causing it. Maybe somebody knows what sorts of things get reset on game load that might affect this behavior?

I don't think I can contribute a lot more to analysing your problem. Uncertainty is frustrating indeed: You play for some time but you can never be sure that your problem doesn't show up any second. It may help to save often (for example, Morrowind crashes occasionally at cell changes, in my mind probably caused by memory management issues).

While it's true that a play session without a problem doesn't tell you anything, a single occurence of the problem tells you that the environment the session is taking place in is "at risk". And, more generally, the laws of statistics provide information, too, in situations like this. In this context: Have you ever seen the problem in an unmodded clean base game environment? If this was the case at least once, it's almost certain that the base game is the culprit. Otherwise, with every minute you play in such an environment without the problem appearing, the probability that it's caused by the base game gets less and less. Which would still leave a potentially high number of mods ...
The other aspect is that the more players are playing a game and/or using a mod, the more likely it is that they observe the same problem as you. This is why I think it's unlikely that the base game causes it. My guess is that it's either a less commonly used mod, or a conflict between two (or more) mods which are not commonly used together (this is why I suggested to try to analyse mod conflicts in my last post).

Finally, if the problem occurs intermittently, but infrequently (like the cell change crash I mentioned above), why not simply live with it? I save my games every 30 or 60 minutes at least, sometimes even more frequently, and accept that I may have to repeat a certain passage once in a while. A matter of motivation.

I agree. If this was a base game problem, I'd expect to find mention of it across the net, but a search yielded no results.

I'm not using any obscure or "odd" mods, preferring to stick with those recommended by mod guides to reduce the chance of exactly this kind of problem. But I do "mix and match" a bit so a mod conflict seems the more likely culprit. Investigating potential conflicts is a challenge with this game, with no xEdit and with numerous mods being entirely script-based. I'm reaching that point. :-) I use Sophisticated Save System with a quicksave count set at 25, so I quicksave often without worry (I've always hated repeating long stretches of a game anyway).

Thanks again for your suggestions. Good to know. But I suspect you're right that these particular parameters aren't related to my immediate problem. (Although my stuck NPC behavior resembles what happens when NPCs get stuck in scenery which can happen with this game, so maybe it is related to clipping?)