Hello! I experienced the same issue and I was able to get around it by installing Xenia Canary through Xenia Manager. By default the settings in Xenia manager default to fullscreen off and was overriding my --fullscreen=true script AND the TOML setting to True setting. It wasn't until i edited it in the Xenia Manager, was i able to get it to load in as fullscreen from the Launchbox interface. Hope this helps! I attached the settings area where i made that update.