It is meant for PC and mobile. That's not good and I would love to help fix that after the next update releases in a week. (＾ｰ^)
What mobile device did you play on? Feel free to DM me. Disappearing characters sounds like low memory, so it will be nice to know what the specs of the phone to target are.
On PC, what browser (FireFox, Chrome, LibreWolf, Opera, etc), operating system (Windows 10, Windows 11, Linux distribution, Mac) and graphics card (AMD, Nvidia, GTX 1080, etc) are you trying on? How much RAM do you have? If you know how to access the console from inspect element, that would be a great help to see the errors there.
Maybe the game really is due for some heavy rendering optimization, not for performance only but battery life too. I test the game on various iOS devices and on a low end computer in google chrome. Android is a blindspot but I should fix that. （╹◡╹）
P.S. There is a downloadable version of FATe written natively for Windows, but it is $7. I will still fix these browser bugs--the browser version is really important.