I guess it makes sense, that an interface is implemented, not inherited, but I think it’s confusing that interfaces are ignored when searching for custom attributes, using the GetCustomAttributes methods.

Attributes shouldn’t be thought of as part of an interface, strictly meta data in that case? No, I think, interfaces should have been searched, that member is part of an interface, and implementations should have access to that members metadata. If nothing else, custom attributes defined with inheritance should be required to be implements by the implementers.