I have a human dual classed Beserker (9)/Thief (10), Gorris, who has 18 CON and is playing under max hit points per level.
At Level 9/9 he had 126 HP (9x14), but at Level 9/10 he has only 132 HP (he should have 134 HP, as he should have gained 8 HP (6+2) at Level 10 for Thief class)?
I also have a Level 11 Bard, Felicia, with 16 CON under the same settings - she has 82 HP, which appears correct, as at Level 10 she had 10x (6+2) i.e. 80 hit points. So this problem doesn't appear to occur for Bards.
I've not checked this for single class mages or single class thieves, which also gain a hit die at Level 10, so can't confirm if they get their constitution bonuses at Level 10. Similarly I haven't checked multi classed thieves.
NB: IIRC, in the original IWD/HoW this was a problem for ALL classes that experience 10 hit dice over 10 levels - i.e.bards, mages and thieves: none of them got constitution bonuses at Level 10, which does not agree with 2nd Edition D&D, or any of the original or enhanced BG games (I think).
PS I've also checked Kirika, a multi class Fighter (8)/Thief (10) with 16 CON under the same conditions - she has 88 HP which is correct, so it doesn't seem to be a problem for her.