Problem combining

I tried combining two golden Leggings with enchants together using a Anvil.

the one with complete durability and Protection IV

the other with 24 durability and Fire Protection IV

but for some reason I can't combine the two at all. 

is this a glitch in the game if so I'd like to get it fixed.

it affects version 1.16.4, but I couldn't choose that version so I added it here