We ran into this issue with our site this past week as well. For us, the situation is as follows:
While most of our categories are intended to be for registered users only, we wanted to have a way for a non-member of the community to make contact with the core developers by email to ask a quick question, raise a concern, etc.
We did this by setting up a category to accept emails from anonymous users using a custom address (e.g., ourproject+contact@discoursemail.com). We made the category’s permissions readable only by core project members to avoid generating too much mail or noise for the community at large (which meant making topics create-able and reply-able by core members as well since there’s no way to restrict read access without also limiting create/reply access).
However, we then learned that if the anonymous user went on to create an official account, they could no longer send mails to the contact email address. This seems counter-intuitive in that they are now only more official and registered than they were before, but can do strictly less than they could before.
This caused me a great degree of confusion before I came across this topic, and still seems like an unfortunate situation.
-Brad