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. 

After connecting one computer from another computer using AnyViewer, you can easily maximize the remote session to full-screen without any obstacles. All you need to do is just click the following icon.  Then, you can take operation on the remote PC as if you are in front of that PC,  

Full Screen