This might be related to #21, however unfortunately is difficult to reproduce.

As I've been using the tool belt in my hardcore world (good usage experience, I will say, removed 4 permanent items from my hotbar), I've seen the durability flicker from used to full, then return all of the tools back to used when one gets used in the belt.
However the tools are in a weird state because of this, and taking them out can cause them to keep their full durability.

I suspect that the new durability isn't properly getting saved while in the belt, leading to this problem.

Don't suppose you have a log please? Don't know if there'll be anything useful in it but it's worth a try

I am closing this since I can't reproduce it, but anyone feel free to reopen