One day, I threw a fire resistance potion on a nether mob that is in lava. After a few mins, it got fire damage!

What I expected to happen was...:
When the fire resistance effect ran out for a nether mob that is in lava, it still survives and did not take any damage from lava.

What actually happened was...:
It started to take damage from lava after the fire resistance effect had run out.

Steps to reproduce:
1. Spawn a nether mob in lava, e.g. wither skeleton, zombie pigman, etc.
2. Throw a fire resistance potion on them. (2:15 mins one is recommended)
3. Wait for the effect to run out for them, it should start taking damage from lava.

Addition
The bug can also be reproduced using tipped arrows of fire resistance.