ToME: the Tales of Maj'Eyal

In The Maze, I have encountered 2 level 25 Slimy Oozes, with a rank of Vermin, which do not die at 0 HP. They have no equipment to give an ability which would allow this (no equipment at all, in fact), and no talents other than Slime Spit. Sometimes they stay at 0 HP, sometimes their HP goes to 'unknown'. The log states that they are taking damage, but they stay at 0 or unknown HP, and attacking has no other effect.

As an addendum, I tried to get rid of one of them by using Animus Purge, which raised it as a husk. However, it still doesn't die at 0 HP, which means than Animus Purge can no longer create husks, since it thinks I still have one, even though that one is still stuck in the Maze, fighting other immortal Slimy Ooze(s).