Not sure if this is an intentional feature that I can fix in the config (I've checked, but there isn't anything that seems like it is related to it) or a bug, but certain gear don't have auto compatibility applied to them. When I have the "Mythic Metals" mod enabled, anything doing 8+ damage doesn't get the auto compatibility. This is the same for diamond armor and netherite armor, without "Mythical Metals". I'll put my current mods down here to see if it is any of them, as some of the items from the mods don't work too.

• Mythical Metals
• BetterEnd
• BetterNether

I should probably also mention that it affects none of the gear included with the mod.