Using Outlook for iOS in business environments for email and scheduling
I have just found I have similar issue. I have a number of IMAP accounts which I use for different purposes. I have recently upgraded my Mac operating system to Big Sur and Outlook to New Outlook.
I have now found that certain e-mails sent from some accounts on my Mac are not synchronising with any of my other devices or webmail. I note that where they are replies though, the mark indicating that I have replied does show up on the other devices!