Sending a mail to a group/category wich allows anonymous mails will work for “real” anonymous mailsenders. But if you are a registered user on discourse but not part of a group wich is exclusively allowed to see/edit a group/category, sending a mail to this group/category won’t work anymore and it will be quitted by a reply that you are not allowed to write in this group/category.
I expected, as an registered user to still have at least the same rights like an “real” anonymous/staged user. So it’s strange that I can not write to an anonymous-allowed category/group wich is secured for explicit groups if I’m not anonymous anymore.
Yes, this isn’t a bug. It’s by design in the current incarnation of the code.
If your email is associated with an account then the posting rights of that account also determine where you can mail in to. The email is resolved against your account, and because your account can’t post there the it can’t create a topic. It would be stranger if you could because per-topic categories aren’t a thing, which means a topic would be created that you can’t see or respond to.
The current solution is to use a group inbox rather than a category for this kind of situation. PMs can have more granular permissions and enable this scenario quite well, we use it extensively.