Huh, don't recall any such loss of XP, but then again been a long time, finished it in 2006 I think? Most likely, remember the period that took place in quite well, but may give or take a year to be certain... Do recall that I won that final battle quite readily. I mean, just have to get one of them down to half HP, or something of the sort? Recall having my cleric take him on and last almost to the end, when a critical fortitude fail before his Black Blade of Disaster disintegrated her, but all others had been ganged up on his sister and just about at the same time got her low enough to win. Did sort of want to do it again to try to finish with all party alive (or at least resurrectable), but didn't care to do it again in the end.
Party was... let's see... Know the cleric was aasimar and the wizard tiefling, both starting with 20 in main stat as a result, and pretty sure they were at level 16 at the end due to the level penalty, with the rest of the party at 17. Quite sure I had a half-orc barbarian, and then recall shield dwarf fighter, I think halfling (but may have been half elf) rogue and I think half elf ranger.