It seems that most apps could not stop the top bar apperaing in fullscreen mode when the mouse cursor being moved to the top area of the screen, of course the Microsoft Remote Desktop looks the same.
It's ignoring when the mouse be moved to the top occasionally.
Tried the Parsec app (https://parsec.app/) , it can stop the top bar coming.
so I hope that the RDP could make sure the same feature in the future release.