I’ve noticed that version 10.4.7 (5309) of the RD client fails to resume properly when you open the app following on from the iOS device being locked or when navigating between apps.
To replicate you start a new RD sessions and then either lock the iDevice or navigate to another app. When you resume the session the session looks valid but when you try to interact with the session there is no response so the only way to resume the session is to close the existing session and start a new session.