This is also an issue with mRemote, I upgraded to mRemoteNG hoping this issue would be fixed. It's actually a little worse.
start RDP session
note lack of scrollbars on bottom and right. This indicates the remote machine and mR agree on the desktop size
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.