Hello, I don't know if this is deliberate or if it has been corrected by now but any charm will lose its enchantments when it is stored in the charm belt.

EDIT : Version 1.10.2-1.3.3.615

that is deliberate and I actually made a change in one of the recent versions to ban the charms from getting any enchantments just to save on confusion

Oh, no mending anymore :,(. Thanks for the answer.

I believe I have put the condition to ban mending when I first released the charms with damage. If not it was always intention for them to not be affected by mending or unbreaking.