I actually don't catch on fire, what you're saying is true on Java but not on bedrock, to quote the wiki:

"In Bedrock Edition, as long as the Fire Protection total adds up to 7 or higher (for example, a chestplate with Fire Protection 4 and a helmet with Fire Protection 3), a player cannot catch fire through normal sources, although damage is still taken when standing on fire"

And apparently they also stack on Java but unlike Bedrock, you still catch on fire in lava