That’s correct, Discourse doesn’t enumerate group memberships.
This is by design. Subcategories can’t have permissions that are less restrictive than their parents. A check was added for this during the recent betas:
Do not allow saving a category if child permissions are LESS restrictive than parent: Subcategories do not inherit permissions from parent category @maja
Source: Discourse 2.3 notes
This is why I asked:
It’s not a category creation error, you’re just using categories in a way that isn’t intended. Had you tried what I asked it would have worked.