Deleting the only new post should make a channel go back to read

Occasionally some servers I’m in have bot spam into every channel. The moderators quickly delete the messages and ban the bot, but every channel the bot posted in still shows as unread. 

Expected result: deleting the only unread message in a channel sets it back to read

Actual result: deleting the only unread message in a channel results in an unread channel with no new messages