I have seen discussions of this for iOS but none for MacOS. My Dock icon for Messages was showing one unread message, but there were no unread messages. I simply removed the Messages from the Dock (drag icon to desktop) and then put it back and now the count appears to be correct.
I have used Macs since 1984, and this is the first time I've noticed this bug, but that does not necessarily mean it is a new regression with OS 26.6.2.
I have used Macs since 1984, and this is the first time I've noticed this bug, but that does not necessarily mean it is a new regression with OS 26.6.2.