There is two ways to fix it.

First Solution:

1. open your qube settings
2. click to advanced
3. on the right you can see allow fullscreen, change use system default to allow
4. restart your qube (if it is running)

Second Solution (if fullscreen mode FPS so bad):

1. start your qube
2. go to video (for this example, i will show youtube.)
3. you can see on the right-middle on the video, picture in picture button.
4. when you clicked new tab is opened for video, now do it fullscreen and enjoy your video as fullscreen

> Bonus solution: You can research "fps problem fullscreen qubes"