When Mythic Metals is installed, weapons made from Promethium do not regain durability. I tested on a fresh install with just Mythic Metals, Simply Swords, a mod to show numeric durability numbers, and all the dependencies. Vanilla tools made from Promethium regained a durability every few seconds, but after minutes the modded weapons did not. It's possible Carmot weapons don't function properly with the extra looting level as well, but that's a bit harder to objectively prove.

Thanks for this report!
I've reproduced this issue on my end and will do some digging.

This is now fixed in the latest version.