Since upgrading to Mojave, I noticed that when I reply to or forward an email, the little icon does not immediately appear to the left of the email, as it used to.
In order for the icon to show up one has to manually switch/click in between inboxes by clicking on them or by quitting Mail and restarting it again.
This is a very annoying bug when, for example, two people are working on the same IMAP email account from different computers. It is actually impossible to see if your co-worker already replied to an email unless you remember to click on different mailboxes to "refresh" them or quit and reopen mail.
In the previous versions of Mail this was not happening.
Thank you for your consideration.
THIS IS MY MESSAGE I SENT TO APPLE FEEDBACK HERE: https://www.apple.com/feedback
Please use this as a template for your feedback. The more people send feedback about this the more chance for Apple to fix it.