When replying to a topic in a restricted category, Discourse (very intelligently) shows you a warning if you @-mention someone who won’t be able to see the topic. It does, however, seem to be a little eager in doing so. For example, if you type
@christine, it’ll warn you that she can’t participate – even if you’re just about to finish typing
@christinejones, who can.
I think this is tied to the autocomplete popup appearing – the warning banner appears at roughly the same time.
One way to fix this would be to not show this warning until you enter a terminating character – a piece of punctuation or whitespace.