Yes, I have checked that Advanced Tooltips are enabled.
Tested with IE 0.12.92 and JEI 4.15.0.293

After smashing a stack of ore, the hammer's durability is at ~50%, but shows no durability number. It also doesn't mend if held in the off-hand while gaining XP.
Also the same case with the Wirecutters:

They're not designed to use mending. This is intentional.

ok, Minecraft did allow me to put Mending on them, so I assumed they'd mend. That's 2 emeralds I lost :P