Thanks for trying it again. So I did some tests are here are my results.
itch.io full screen button and Click to launch in fullscreen works fine as it keeps the ratio as long as the Unity full screen button isn't pressed.
Unity full screen button causes the screen to stretch causing some behaviours that weren't accounted for.
In the end, it seems like Unity full screen button should be removed so no one clicks on it but I can't reupload it for now.