Can't get rid of new response notification for deleted question

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I get a response. My mailbox symbol is red. I click on the response.

Sorry, the comment you are looking for is no longer accessible, because the parent question has been removed

I can do this many times, but the responses are there and shown as not seen yet. I have to explicitly mark them as "seen" before the red mailbox becomes white. This seems to be a bug. In fact, presumably things from deleted questions shouldn't show up at all, though they frequently do.