Very disappointed to report that the Fire Protection enchantment in Windows 10 Bedrock 1.18.12  (most likely in previous versions as well) seems to be more or less completely off.
Standing in lava with unenchanted diamond boots or wearing an unenchanted helmet gives me a lifespan of about 6 seconds. Enchanting these boots (or helmet) with Fire Protection I, II or III makes zero difference. With level IV FP my lifespan then changes to about 11 sec.
Wearing all full set of FP enchanted diamond armour III or IV leaves me completely unharmed until the armour itself breaks. I don't think this is as intended either.
There are too many combinations to test. Here are just a few others:
Wearing boots AND helmet, enchanted with FP I : 11.5 secs
Same combo enchanted with FP II : The same! 11.5 secs
Chestplate AND boots with FP III: the same! 11.5 secs
Wearing a chestplate only, also gives me about 11.5 secs lifespan regardless whether it is enchanted with FP I, II, III or IV !!!

This makes me wonder if the other armour enchantments are equally as broken.