Pickaxes are inconsistently applying mending.  With no other mending items in toolbar or equipped that are not already at full durability... and a netherite pick that is in the red, it appears that only the first orb mends.  The remainder go to the XP Bar.  This tracks with the visualization of the tool on the hotbar... it does not "jitter" like it would when being mended.  I've sometimes had luck switching off the tool and back but honestly it seems more time based than toggle based.  Didn't notice a similar problem with diamond tool but honestly haven't had time to test yet.