Titof68: My characters are not getting the amount of hit points the rules dictate. For instance, my main character, a Fighter with a Constitution of 15, thus with a +3 HP/ level, has just levelled up to Lvl 12 and only received 3 HPs! That shouldn't even be possible. I reloaded and tried again with Easy Difficulty (so as to theoretically get Max HPs) and still I get only 3 HPs. The same happenes with other characters.
Has anyone else run into a similar bug?
It's not a bug. In AD&D (both 1e and 2e), characters stop getting random HP rolls at high levels and instead get fixed HP bonuses at level up, which are not affected by Constitution.
For example, a Fighter who levels up past level 9 gets 3 HP per level, regardless of their Constitution; hence what you've observed is, in fact, correct behavior.
Also, note that 15 Constitution only gives you +1 HP per level, so even at lower levels, when you roll for HP, 2 HP would still be possible. For whatever reason, in 2e you need to have a really high or low ability score for it to actually matter; 14 Con is no better than 7 Con here.
(3e changed these mechanics, giving you new HP rolls at higher level, and giving stat modifiers to those with near average stats.)
In other words, it's not a bug, but it's rather the mechanics being different from what you're expecting.
