Flathub shows the OT version as 1.4, even though 1.5 was recently released. The flatpak for OT is owned and maintained by community members and not maintained officially so I don't know when it will be updated nor why it installed 1.2 for you. Either way, it appears you are running into the display issue with 1.2 and you probably need to use a more recent version.
I'm not 100% sure if it will run on Chromebook, but try downloading Linux AppImage of OTX 1.5 (a variant of OpenToonz) and see if it will work for you.
Pick the release and download OpenToonz-OTX-linux.tar.gz.. Unpack it (unzip or whatever to open it), move the entire OpenToonzPortable folder somewhere on your machine, like your Desktop, then run the OpenToonzPortable/OpenToonz.AppImage