Interface &1 still contained in interface &2

What causes this issue?

You wanted to delete the implementation relationship for interface
&V1&. However, interface &V1& is still implemented in interface &V2&.
Since all interfaces contained in &V2& must be implemented, you cannot
delete the implementation relationship for &V1& unless the
implementation relationship for &V2& is also deleted.

Error message extract from SAP system. Copyright SAP SE.