Netherite armor on realms seems to burn in lava or fire. I tested this in multiple backups and it seems to always burn like any other item. I then proceeded to test in a single player world and it would have normal behavior (bounce around until it makes it's way out of lava).
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We're tracking this issue in MCPE-84796, so this ticket is being resolved and linked as a duplicate.
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This is probably a duplicate