Summary: When attempting to inherit special upgrades from one weapon to another without a gem upgrade.
Difficulty: Normal
Occurrence: Always
Details: I figured out my issue, but I'd like to bring this up because it confused me for quite a bit and might help other players! I was trying to special inherit my Special Upgrades from my Demolition staff to my newly created Lord of Ruination staff. There was no indicator that I should have gem upgraded my Lord of Ruination staff (though it feels kind of obvious in hindsight) and spent a bit of time trying to figure out if what I did was wrong. Until I figured out I didn't have a gem upgrade on it.
I don't think this is a bug so much as a request to just be able to label/indicate if something is missing if you're unable to inherit! As you can see in the screenshot, the source equipment has special upgrades but the target one did not. I think I was also confused there.