It's not unusual to meet RDC session window full screen not working. Apart from taking measures to fix the issue, you can directly resort to the great third-party remote access software, AnyViewer. It works well under Windows 11/10/8.1/8/10 and Windows Server 2022/2019/2016/2012 R2. Now, it also supports you to access computer from iPhone, iPad, or Android devices. 

After remote access to one computer over AnyViewer, you can easily maximize the remote desktop window to full screen without any obstacles.  You only need to click the full-screen icon on controller toobar.