Huh, okay, the fix itself should be simple. To run the game on joiplay you need renpy plugin 8.2.1 or above. That's it.
What's weird though is that I barely could find it myself. If I just searched "joiplay renpy plugin" it always proposed me 8.1.3 calling it the latest version (including their official site) when that's not true at all. The newer version has been released for a while now and I'm pretty sure it was easy to find before...
Anyway, you can find the current latest versions of joiplay and plugins on their patreon (free post): https://www.patreon.com/posts/joiplay-1-20-530-109444415
Just download "JoiPlay 1.20.530" and then "Ren'Py 8.2.3 Plugin 1.00.40".
There is also might be a probelm with renpy plugin not seeing the main app and so not dowloading. In that case:
1) Open JoiPlay and tap the tree dots on the top right
2) If the JoiPlay version is 1.20.500 or above, there should be "plugins" button there.
3) Go to plugins and find "Ren'py 8.2.1". Dowload it.