It shows as fullscreen no borders or anything but the UI doesn't give me Full Screen as an option. Windowed and Full Screen Windowed look the same to me. The option in the settings always resets to Windowed when I restart.
Hmm, that's strange. To be honest, I don't know what could be going on, I'm sorry, and I don't have a Linux machine to test it on. The tester I sent the build to didn't report that error.
If it won't let you go full screen and you can only play in windowed mode, maybe that's where the problem lies, but I'm not sure.
There are many other problems that could be the cause. 
Can you see the text but not the videos?
In the case where I try to play the video, the game can't proceed past that point, it just stays on a black screen. So unfortunately, that route is effectively blocked for me on Linux.
It's okay, though. I'll probably try the Windows version through Proton at some point to see if the video plays correctly there. I'd like to see how the content changes when the Lust points are higher, since skipping the video lowers the Lust points.
For now, though, I'm going to take a break from it rather than keep troubleshooting. :) Thank you for taking the time to help me debug it and for all the suggestions. I appreciate it.