Ok. The solution is like suggested on the micronaut docu (https://micronaut-projects.github.io/micronaut-test/4.0.0-M8/guide/index.html#junit5) Instead of using @Spy on all "normal" implementations. I just had to use

    @MockBean(ClassToBeMockedImpl.class)
    public ClassToBeMocked classToBeMocked() {
        return mock(ClassToBeMocked.class);
    }

on the class I want o mock. So all the field injection were uncessary.