While trying things around as a Vampire Lord, I noticed that Mist Form didn't really make health regenerate at all, and looking at it it seems the problem is a jumbled mess due to how conditions are used in all the relevant magic effects.
The spell
DLC1VampireMistform uses several tailored effects, but all of them are checking that the player is not under any of the effects DLC1VampireSlowTimeEffect, DLC1MistformEffect or DLC1BatsEffect before they can take hold.
DLC1MistformEffect is the first effect in Mist Form's list, and since all the others are checking that it not be present, it's the only one that takes effect, making the player ethereal. Regeneration, invisibility and muffling effects fail to work. The stamina regeneration effect checks for nighttime hours in the spell for some reason.
DLC1MistformInvisEffect is not an actual invisibility effect archetype and lacks the relevant keyword. The various effects also use a magic school in some cases
I've removed from each effect the condition check for the spell they come from, which solves the issue with Mistform. I removed magic schools, fixed the invisibility archetype, and also gave the bat absorb health cloak ability a check for not being invisible, since it alerts enemies otherwise.
Maybe I overreached as usual, but at the very least I'd urge to look into correcting Mist Form so it can work as the perk describes.
Attached Files:
Vampire Lord Effects Fixes.zip
Issue Data
Issue #25877: Mist Form doesn't actually work as described