The bug
Fire Protection's burn time reduction is not applied if you are already on fire when you start wearing it.

Steps to reproduce

1. Place down some lava
2. Give yourself fire resistance (so you won't die)
3. Wear 0 pieces of Fire Protection
4. Go in survival mode
5. You'll be on fire
6. Start wearing two pieces of Fire Protection IV (should make you stop burning instantly)
7. Step out of the lava --> You burn for way too long (as if you are still wearing 0 pieces)
8. Get back in the lava once you finish burning
9. You don't even get set on fire (apparently this is the way it works on Bedrock, and is probably intended)

Expected results

Putting on Fire Protection armor when on fire should reduce burning time

Observed results

Equipping Fire Protection whilst burning doesn't reduce burning time