As a GM, I use some revenants for my campaign. Those revenants all have class abilities and 2 are spell casters (an abjuration mage and alchemist artificer). The "problem" (it's not a problem, but an "and what if they do that?") is that they can use the Protection from Energy spell, so they can have fire resistance.
They have a Regeneration feature, with the following rules:
Regeneration. The revenant regains 10 hit points at the start of its turn. If the revenant takes fire or radiant damage, this trait doesn’t function at the start of the revenant’s next turn. The revenant’s body is destroyed only if it starts its turn with 0 hit points and doesn’t regenerate.
My question is: What happens if they gain fire resistance (or radiant resistance)? Would they lose the weakness that prevents Regeneration from working when fire damage is dealt?