How do I deal with Steam's phantom comment notification?

There is a phantom "new comment" notification haunting my Steam account. You know, one of these things that’s tracked by the wdiget with the envelope in the upper right corner of the window in the client or the website:

Before I try to view it, it appears to be a regular comment notification. When I get X new notifications, the overall notification counter shows I have X+1 new notifications; and when Y of them are specifically new comment notifications, the menu says I have "Y+1 new comments". All counters tick down properly as I read up the notifications – at least until there’s only the phantom notification left.

When I try to view it, I either:

• (if there have been other comment notifications recently:) get a list of new comments that accounts for all the notifications – except the phantom one;
• (if there have been none) get an error screen assuring me I have no comment notifications at the moment, like this:

I’m assuming it was created when a vitriolic user made a post in a Steam forums thread I was following, and became phantom when that post got deleted in some specific time window between me receiving it and trying to view it.

I have had this phantom comment notification for about 5 months now. It didn’t go away after I received and clicked several newer comment, item, or Steam support notifications.

It persisted across daily log-ins, daily log-outs, daily PC boot ups, and many, many Steam updates.

Steam support doesn’t know how to deal with this.

I’m assuming doing anything with the client won’t change anything, since it also haunts the website.

Viewing notification in Big Picture mode didn’t help at all.

I’ve tried scrolling all the way down to the [My Username] > Activity log. I’ve reached the bottom – no fix.

Is there any way to make it go away?