When killing mobs while holding a mending tool, the experience that is dropped from the mob will not be added to the experience bar, but rather will be taken by the mending tool/armour. This will keep happening until the durability is full or if you use a non mending item

UPDATE: Only one item will be mended at a time if multiple items are damaged

>>TESTED PLATFORMS<<
Me: Android- Issue occurs
My cousin: Xbox- Issue occurs