I cannot reproduce this when I post in a category that is exclusive to certain members (e.g. the “Staff” category).
But, at least in @icaria36’s case that’s not what happened. The topic was created in the “Site Feedback” category which is visible to everyone. I guess the fact that Discourse sent a notification was unexpected because the forum is configured as login required.
I’m not sure if we should prevent notifications emails to staged users when the forum requires a login (and maybe even is invite_only or requires user approval). I’m on the fence here. The notifications certainly make sense when the forum is public.
@dachary and @hellekin: Can you please verify the permissions of the “private category”? I don’t see a way for a staged user to get a notification unless the user has access to the category.