Unsure if this was an intented change, so I'm reporting just in case.

Fire protection special attribute generic.burning_time can now stack for every piece of armor, instead of only applying it for the armor piece with the highest value (highest level enchantment).

This might be expected as per the fix for MC-271573, however there's the side effect of burning time being completely mitigated by the player wearing gear with a total of 7 fire protection levels.

Note: According to the minecraft wiki, this behavior is apparently how it works on bedrock edition, however I am myself unable to confirm if that's the case.

How to reproduce

1. Enchant a helmet and a chestplate with fire protection IV.
2. Equip both pieces of armor.
3. Place some lava, walk into it.
4. Notice how apart from the lava damage, the player doesn't catch fire.