"High priority" flag ignored on message send
Create a new message and press the "Set message priority to high" icon (or Options->Prioritize Message) (as per these instructions https://help.gnome.org/users/evolution/stable/mail-composer-priority.html.en) and then send the message. The receiver will see this message as normal priority (instead of the expected "High priority"). This was tested via an Office365 account as the sender via evolution 3.28.5-0ubuntu0.18.04.1 and 3.32.4, sending to various accounts (Office365, msn, gmail).
Also, it does appear that at one time there were "Send" options specific to EWS that allowed you to set the message priority: https://help.gnome.org/users/evolution/2.32/exchange-settings.html.en#b15flgn7
These do not appear to be available anymore, not sure if this is intended.