When I press “Play Game” the game forces itself fullscreen on me, but its being displayed properly. I can guarantee it’s not an Itch issue, because I have a game made with PixiJS myself, and had no issue displaying it. My guess is there’s something wrong with how the game is configured?
In case it helps, my game is this one: https://darkdimension.itch.io/the-collector
It’s open-sourced here: https://notabug.org/tomtsagk/collector_game
It is using a PixiJS engine I’ve made, it’s also open-sourced here: https://notabug.org/tomtsagk/pixijs-game_engine
The only reason I’m linking these, is so that maybe it will help if you see how another project that works is configured. This is a very old project of mine, so I’m sure it’s not perfect, but maybe you can copy the needed parts to make it work.
Also please don’t make the game force itself full screen as it starts. Make a button to toggle fullscreen instead, it’s much better for users.