You can still take fire damage even you have fire resistance on

I was fighting blazes for their rods (I drank a fire resistance potion). When I was fighting them, I accidently walked into fire and start taking damage from it (not burning). And I got knocked off by blazes because the fire blocked my screen.

What I expected to happen was...:
When I walked into fire with fire resistance, I won't get the damage from it.

What actually happened was...:
I start taking damage from it.

Steps to Reproduce:
1. Drink a fire resistance potion. (mine was the 8 mins one)
2. Walk into any bodies of fire
3. Notice that you start taking damage. (this won't always happen)