With test user(TL1) open the channel. (just to be read)
After you read it go to an other page or close this tab.
Move these messages to an other channel with staff.
Now back to user(TL1).
Reload the page or open if you closed this tab before and try to open the Test1 channel. The channel seems empty and the following error appears. The unread state stuck. (The keyboard shortcut SHIFT + ESC can fix the channel.)
It seems when you not reload the page before open the channel the system message appears and a simple reload enough to solve the issue.