FYI.
Another workaround, if "max cached connections=1" is not practical in your environment, and if you are eager to know flag/keyword change of existent mails by other mail client even though your IMAP server doesn't notify flag/keyword change to Tb via IDLE :
Go "Work Offline mode", then go back to "Work Online mode",
and, if folder you want to know flag/keyword change by other is FolderX,
click an account or other folder, then click FolderX.
FYI.
Another workaround, if "max cached connections=1" is not practical in your environment, and if you are eager to know flag/keyword change of existent mails by other mail client even though your IMAP server doesn't notify flag/keyword change to Tb via IDLE :
Go "Work Offline mode", then go back to "Work Online mode",
and, if folder you want to know flag/keyword change by other is FolderX,
click an account or other folder, then click FolderX.