This is also an issue with mRemote, I upgraded to mRemoteNG hoping this issue would be fixed. It's actually a little worse.
- maximize mRNG
- start RDP session
- note lack of scrollbars on bottom and right. This indicates the remote machine and mR agree on the desktop size
- minimize mRNG
- restore mRNG
- now one or two scroll bars appear. mR and remote machine no longer agree on desktop size
- on classic mR the following workaround can be used to restore proper window size: 'restore down' and 'maximize'. This does not work with mRNG.
Thanks for looking into this. mRNG is terrific besides this small issue.