When watching "All" at Confluence questions, I am notified by email about questions that have been asked in spaces where I do not have access to. Both links to the question itself and to the respective page are giving me an error "page not found". In my opinion a notification only makes sense for users with access to the question. It is also critical as we are using questions for confidential topics where even the question title which I can see in the notification should not be visible to anyone without access.
I am not able to reproduce the issue; I believe that was fixed already
Out of curiosity, which version of Confluence are you on ? cloud ? server ?
The latest similar issue that I am aware of, and which also got fixed recently is https://jira.atlassian.com/browse/CONFSERVER-52763
I hope this answers your question.