Yes, on 5.5.4
I just spent all afternoon building a material function, reopened the project and I’m staring a this:
Edit: After rebuilding from memory because now I know what I needed, I would probably suggest that if dealing with material functions, copy it to a safe location just to make sure. Nothing else appears to be affected but I filed a bug report anyway. Regardless of whether it’s replicable directly, there’s obviously an issue that needs at least filing.