The altars in the Temple of the Moon should give +5 to stats to each party member. For some reason the luck altar isn't giving the full +5 bonus to all of my characters. Is it a bug or is there some kind of stat cap with these?

I always did the altars in the wrong order and didn't get the bonus. Didn't even know there was one until later. I think you need to have the altars reset, touch the central sign, then the altars. But again, I always screwed it up. Not so much a bug as a need to do the correct order, I think.

I've done the altars on correct order so that isn't the problem. I tried doing the temple on earlier save too with the same results. It seems the luck altar is indeed bugged. It only gives +5 luck to 3 out of 4 characters. The 4th character only gets +2 to luck. Pretty minor stuff and I only noticed it because I'm rushing some stat bonuses early on this run. Wells>Black pots>Temple and your whole party has ~30-40 in all stats by lvl15.

The game is great fun after all these years, there just seems to be lots of incorrect or hard to find information about the actual game mechanics. For example paladin/archer have starting mana multiplier of 1, not 2, which makes them pretty bad casters early, and meditation is absolutely useless for them until much later in the game.

If it only gave two points (which seems like a strange bug) could be a cap that exists?