XP is dividing itself when it should not

When getting XP to repair my mending tools the XP does not go entirely towards my broken tool but instead goes partly to my tool and partly to my XP-bar.

 

After some experimenting I found that the cause of XP dividing is not due to me wearing mending armor (I checked this while this was a previous bug).

 

As proof en explanation I have added a video showing the bug.