Welcome to GOG. Purple dot notification is broken for years.

You can see working notification when someone actually responded on community page (it'll appear above wishlist box). There are also listed topics you recently participated in.

Also if you post in a thread which gets deleted it will nuke your thread history viewable from the forum front page.

How to nuke the purple dot:
https://www.gog.com/forum/general/heres_how_to_remove_permanent_forum_reply_notification/page1

Worked. Thank you!

The reason for this is a simple but stupid mistake.
I firmly believe that the "My posts" page's database query works like this: it filters all comments you ever wrote, from there it discovers all threads you participated in, and then orders those by date, filtering any that are older than 1 week.
The problem is in the source of the date of that ORDERING. Instead of grabbing the last comment you wrote in any thread it picks the first comment you wrote. Which means that, if you post in any thread, after one week it will never again* appear in the "My posts" page, no matter when or how many comments you write after that.

* Except if someone hit the Reply button on your comment and posted something, which flags that comment to also appear in "My posts" until viewed by you once. This flag is also the one that causes the "arrow in a grey gear" icon to appear in the forum thread list. The reason for the "eternal purple dot" is unknown but if there really was a reply waiting for you to read you'd be able to find it because of that flag. At least it can be solved like ConsulCaesar showed you.

The reason no one fixes the stupid "My posts" behavior is that no one noticed it back when the forum was new and the people who created it worked at Gog. Now that those people are long gone everyone is afraid of touching this hot mess kept together with spit and duct tape for fear of it all crashing down. There must be a very good reason for Gog to order their coders to take that risk, (like a pronounced loss of money) and as rightfully annoyed as you and many of us are it's just not enough of a reason.

It's been two days since you replied. I've read it four times now, because the "notification purple dot" does not disappear. Must be at lease one more stupid mistake on top of the one you mentioned.
So I had to do the
angular.element(document.body).injector().get('menuNotificationsRepository').deleteAllNotifications();
thing again. See screenshot.

You are correct: that is indeed a different bug. Who knows what is causing the purple dot to stick around when it should get cleared. I just use the same trick you do to get rid of it.

The effect caused by the mistake I mentioned is only that a thread where you have written recently won't appear in the "My posts" page because one time in the past (at least a week prior) you had written in that same thread.

Yes, the purple dot will reappear whenever you get a reply. That's how it is supposed to be and works exactly as designed.

However, it should disappear once you have viewed the reply and that's what currently doesn't work. So you need to run the above javascript line every time after getting a reply.

That's why it's best to make a bookmark for it so you can clear the purple dot just by clicking the bookmark.

On top of that, as for my initial complaint: The purple dot sometimes goes away, reappears, and you seen no forum reply.
When this starts to happen more often I map an autohotkey macro to a keyboard shortcut :D.