fire protection bug

when i wearing a armor with all fire protection ii, iii and  iv 

one day i found a diamond inside a lava, but accidenly i fall into a lava but i swim up. But i dont get any fire on body (when i fall into a lava i dont get any fire particle, i wearing a fire protection iv)

but i get my helmet and legging, still dont get any fire particle. but i wearing only chestplate with fire protection iv i get a fire particle in my body (i test use a fire protection i , ii and iii i still get a fire particle with full armor) but fire pro-tion iv i dont get any fire particle only damage only (but i fall in lava, my heath dont damage repeating in lava, but damage like drowned in water only)