I use Mint to test the version, though I've had someone else test it and they were on Ubuntu. It fails regardless of installing media packages like gstreamer.
If you wish to know the technical reasons, the framework I use serves assets on a custom protocol. The game runs on web views and on Linux this is webkit2gtk. Recently (several months ago by now), this web view released an update that broke mp4s served on that custom protocol. I'm not sure why or what happens but it is a known issue.
If you can, send me a message on Discord and I might be able to send you a build for you to test.