I have 2 related bugs that I think are fundamentally the same core issue of death speech scripts being both 1 time events and reliant on things other than the hp of the enemy hitting 0 (or 1, whatever it takes). If the script fails to go off the first time, it will never try again, and thus the enemy will be permanently unkillable.
The first bug relates to Abazigal. It happened twice and is thus replicable with my playstyle, but I was streaming and couldn't test to differentiate between two possible options that could have broken it. I'm not sure if it's because the MC was mazed, or if it was because there were no valid party members in Abazigal's line of sight. Specifically what seemed to trigger it is that I killed Abazigal via time stopped meganuke while the main character was mazed and nobody else was in melee. The meganuker was likely out of line of sight by the time the nukes hit.
On my third attempt (the successful one) I ended up using the same strategy but waited until the MC was not mazed, and my mage was not effected by the casting failure. This led to killing him while he was in the MC's sights, and his script triggered normally despite a much heavier meganuke (Neera cast roughly 6 dragon breaths along with a dozen or so other aoe nukes during the timestop due to lucky nahal's usage). Due to this I find it unlikely that the bug is related to overkill damage.
Current behavior: If the Main Character is mazed and/or no valid characters are in Abazigal's line of sight when he reaches his death speech threshold, the script fails, and cannot be reset even if you heal him back to full life and beat him back down to death range. Thus, he becomes incapable of dying and you need to reload.
Expected behavior: Abazigal does his speech regardless of mazed protagonists and/or line of sight to the party. If this isn't possible, the script should continue trying to fire any time he takes damage while at death range. To prevent chain firing the second option would require an internal timer.
The second time I ran into the same form of bug related to Korkorran in Hexxat's ToB quest.
Current behavior: If Korkorran is fleeing due to turn undead when his death script is meant to fire, it does not fire, and he becomes permanently unkillable. He also seemed to become permanently turned. I left him standing there without turn undead on for several minutes without the effect dropping.
Expected behavior: Turn undead does not turn off death speech scripts.
Since I was streaming these events I can probably create videos out of the fights if it'll help diagnose the problem. The Korkorran one was pretty straightforward, but the Abazigal fight was much more hectic and confusing.