Summary:
Fire resistant mobs (warden, strider, zombified piglin, and wither skeleton, but not blaze, magma cube, or ghast) have a burning animation in a lava cauldron. This also applies to mobs with the fire resistance effect. Mobs don't take damage.

Steps to reproduce:

1. Build a 1 block pit.
2. Place a lava cauldron nearby.
3. Summon the fire resistant mob in the pit.
4. Push the lava cauldron with the piston in the fire resistant mob.

Observed results:
Fire resistant mobs inside the lava cauldron has a burning animation.

Expected results:
Fire resistant mobs should not have a burn animation.

Image: