It seems to be consistent with losing the temporary HP. Also, the regen applied by berserker rage doesn't seem to work, as I'm not gaining any HP. (Maybe because of weird interactions with the temp HP)
Through a whole rage, I got 1 hp regen, when the rage ended. Another time I had 0 regen at its end. Otherwise, 0 regen during raging.
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Maybe if the temp HP is applied separately, it'll work better in the case of Malkizid. As Tenser's transformation spell mentions that the temp HP there is applied separate, because of otherwise premature ending.
"The temporary hit points are applied independently of the polymorph, so these hit points can persist if the polymorph is canceled before the spell expires. (The reason for this is the reverse situation — losing the temporary hit points through damage will not cancel the polymorph.) This stacks over multiple castings."
https://nwn.fandom.com/wiki/Tenser's_transformation
Maybe this'll help with the no regen during berserker rage too?