Why is RestrictingType<?> not within the bounds of T?

Because ? is an unknown type because T is actually not bound in RestrictingType<T>. It can be anything, like Integer. Therefore the compiler cannot know if ? implements RestrictingType and consequently SelfReferringInterface.

And why is the functionaly similar ? extends RestrictingType<?> no problem?

Because now you've bounded ? with RestrictingType<?> which means compiler knows it's a RestrictingType and consequently SelfReferringInterface.

Also, funny, but this compiles and runs on my Java 21.