For all of those who did not add a CollapseCategories property, my detail pannel bugged out, because I typed into CreateDefaultSubobject the same name as the root variable name in the parent class.
Just changed that:
RootSceneComponent = CreateDefaultSubobject<USceneComponent>(TEXT("RootComponent"));
RootComponent = RootSceneComponent;
…into this:
RootSceneComponent = CreateDefaultSubobject<USceneComponent>(TEXT("RootScene"));
RootComponent = RootSceneComponent;