Compiling a class with a nested interface results in a reference to a non-existent inner class "X$Y$1" in the InnerClasses attribute of the resultant class file. Nested classes and inner classes do not exhibit this problem, only nested interfaces.
Sample class that demonstrates the problem:
Examining the resultant class file:
There is a reference to "X$Y$1" which does not exist. This breaks some tools, for example JUnit 5 test discovery.
It looks to me like this is related to https://issues.apache.org/jira/browse/GROOVY-5082, being the same bug just manifesting for nested interfaces.