For some reason, Constantan is using the material properties of Invar, and doesn't have its own properties. I found this out when tweaking Invar and Constantan.

I am on the latest 1.20.1 forge version, 3.2.1.

Are you, by chance, attempting to change what Weapon Traits exist in either material?
If so, I think I found the offending line:

Oops. I'll get that oversight fixed in the next update.

Yeah. I had the bright idea to rename Invar to Titanium, since I don't have Invar in my modpack. So, I added Lightweight to it, and realized it was showing up on Constantan (which I renamed to Dawnstone).

It's no biggie. I am not even sure those materials are used nowadays in 1.20.1, so I might be the only person who ran into it.