Usually when an item in Minecraft had durability, a bar is shown under that item which goes down and turns a darker color as you lose durability. In addition, there is usually a number shown as ?/? in your inventory when you hover your cursor over the item. This can also be fixed by mending when you gain experience points.

I still had the durability bar under the item but did not have the durability number, and mending did nothing to repair my items when I had on armor, but did repair when I had no armor. This is applying to all my items with durability on all my worlds.

I'm not sure how the bug was triggered, one day I had the durability bar and the mending effective, and the next both were gone.