Dual-Class HP Special Case Error CON Bonus Not Applied

Emptiness

Level 9 Cleric (Priest of Talos in this case) with 16 CON dual-classed to Mage. Upon advancing to level 10, the character should gain 1d4+2 hit points (so 6 at Normal difficulty). Instead, the character only gains 4 hit points. I suspect that the CON bonus is not being applied, either because the game mistakenly believes that CON bonuses are only granted for the first nine levels (which is not the case for Mage/Sorc/Thief/Bard) or because the game mistakenly believes that this character is only supposed to get a CON bonus for the first nine levels due to being a Cleric (which is not the case, because the CON bonus in question would be being granted for a Mage level).

Assuming this error is restricted to this specific situation (dual-class at 9 from a 9HD class into a 10HD class), it is not very serious because affected characters will never be more than 2 hp short of their intended total. If this can be fixed easily, however, then it should be because on higher difficulties characters will not be getting max hps at each level and every hp counts when the monsters are hitting for extra damage.

Aramintai

There is something wrong with hitpoints at level up, alright. Most of my party members somehow are getting only 1 HP per level, even though some of them have 17 CON and are fighters. I even checked old save with same party at same levels in vanilla BGII - they had considerably more HP.

Mathsorcerer

Once you dual from cleric over to mage the game registers your class as cleric_mage, not mage. This class gets its hit points from hpprs.2da which, at level 10, give you a straight 2 hit point plus your constitution modifier for a total of 4.

Multiclass characters will still have their straight hit point additions divided by 2 for every level up, so fighter/mage will get either 1 or 2, depending upon whether the game rounds up or down (I am uncertain which one it uses), not counting constitution bonus (which also gets divided, I think). I would expect a fighter/mage of sufficiently high level and 17 constitution to receive 2 hit point at every level-up, regardless of which class is leveling.

Emptiness

That L9 PoT/L10 Mage has 94 HP in game after completing the leveling process. Opening up the character in EE Keeper, Base HP is reported as 76. CON bonuses are not stored directly, so we can see that the game is giving 94 - 76 = 18 hit points from CON, which is 2 hp/level for 9 levels. The HP awarded at level 10 was not 2 + 2 CON bonus. It was 4 + 0 CON bonus.

In either case, the point here is not so much how the incorrect amount of additional hit points is being calculated, but rather the fact that the amount of additional hit points is incorrect.