Forum channel keeps its unread indicator when the new thread has already been viewed

You will need 2 accounts for reproducing this since it relates to the unread indicator.

1. Create a new forum.
2. From account 1, create a post in the forum and ping account 2 in it to add it to the thread: pasted-image-1.png
3. From account 2, open the thread you were pinged in by clicking on it in the channel list (NOT on the forum channel): pasted-image-2.png
4. See that once you've read messages in the thread, the forum keeps its unread indicator even though there is nothing new to see anymore: pasted-image-3.png pasted-image-4.png

I expected the unread indicator to disappear once I've seen the new thread.

The unread indicator remains to be shown on the forum channel.

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