Installing the game through GOG, everything seems fine until I get to the mansion. When I try to put out the lights, nothing happens to the lightmaps: they stay the same as they were, as if the fire is still going. I tried a full TFix install and it didn't help. Also, just to make sure, I installed the game through Steam (no TFix) and the lightmaps work fine, and I played Thief 2 from GOG (with newdark) and the lightmaps work fine there as well.

pcgamingwiki helped me answer this. I must have installed TFix on top of Gog's version without restarting the mission, which is the source of the problem.

it's simply how things work - after patching up, all the new resources need to be fully reloaded to initialize properly, and only a complete mission restart will do that.

also, this has been answered many times in the past, might want to google a bit next time (TFix light problem).

Sorry. Thought I was doing the google thing. I also thought I restarted the mission when I encountered the problem, but I must not have.

Hi .o/ I'm wondering, would it be possible for whoever is maintaining NewDark to modify the engine so that it can do that every time you load a save (or at least if no mission is already "loaded" as in, the user has started playing)?
Or am I misunderstanding what it really means and the existing save would still be unusable/not 100% compatible with those new resources, which would still cause glitches?

probably not - it's just like when your os updates its system files, and needs a reboot to start using them.