Converting TODO-NULLABLE into the issue here since I'm not familiar with the area I'll leave it for area owner to decide if they want to close it or leave it. I'll remove the original comment in case it's decided this should be closed.

In ValidationAttribute.SetResourceAccessorByPropertyLookup (last line):

_errorMessageResourceAccessor = () => (string)property.GetValue(null, null)!;

Is commented as If the user-provided resource returns null, an ArgumentNullException is thrown - should probably throw a better exception