Noticed this happening since 1.14, dont know if its happening in earlier versions as well
When you have mending on both your armor and the tool you are holding, sometimes the gained experience doesnt repair any of these but just levels you up. If i remove my armor, my mending tool on my main hand gets repaired correctly with the gained experience. but if i have 2 items on both my main hand and offhand, both of them with mending, sometimes they get repaired correctly and sometimes they dont.

Pretty much happens the same wherever you get the gained exp, killing mobs, fishing, mining, or smelting.