Board Thread:General Discussion on Baldur's Gate Game/@comment-91.219.12.16-20180501062730/@comment-3234315-20180501162822

Just tested this in the original game, not the Enhanced Edition, though with a true kit fighter – shouldn't make a difference. It happens there also! So it seems, the Constitution bonus isn't calculated anymore for dual-classes that have exceeded the first set of Hit Dice rules …

The PlayItHardcore wiki notes this: "'If you dual-class into Fighter, you will only earn the Constitution bonus until the SUM of your two classes' levels exceeds 9. You will then lose the bonus and start gaining 3 HP per level statically.'" Which confirms what you (and me) observed.

The Player's Handbook states: "The Constitution bonus ends when a character reaches 10th level (9th for warriors and priests) – neither the Constitution bonus nor Hit Dice are added to a character's hit points after he has passed this level." Well, the Hit Dice are mentioned as well; a more detailed statement about dual-classes isn't made.

Is this an oversight in the rules?