I suspect this must already be reported, but I can't find it.
Gave Black Pits 2 another go, since my first party failed with the crash-dualing-druid-to-fighter bug, and the cleric/thief's inability to access their lock picking ability (my only thief).
So this weekend I created another party in BG1, and took them all the way through the black pits. A key character was going to be my first Kensai/Mage dual, who hit level 9 Kensai just before our fight with Irenicus, so I dualed and kept her safe in the final fight, so I would gain the benefit of the xp bump on import. (Foolishly, I let this overwrite my earlier saved game).
On import, my KenSage with 18 con has only 40 hp - less than if she had rolled 1 for every hit die. This makes the character largely unplayable for me - my straight sorcerer imported for the same party with 2 less con has 50% more hit points (60) and did not give up the ability to wear mage robes, or a specialized per level, to get them.
I tried importing this character as a regular BG2 game, to see if I could get a fix and import that way - but the same import bug affects the main game.
I'm not sure how these hit points are calculated, but it looks like they are not even accounting for Con in whatever algorithm is used, never mind picking the wrong hit dice.
In case it is relevant, I am running the unpatched version of the game from the Map App store.