[Desktop Web] "Mark Unread" does not work on username mentions on old reddit

I use username mentions to help me mod my sub: people use a username mention to point out rulebreaking behaviour.

As I mod 440 subs, modmail is a bit messy.

If I cannot deal with that request immediately, I mark it as "unread" and move on, to be dealt with later.

However, that comment never returns to my inbox, so it gets missed.