I would suggest closing all other apps before running Diablo Immortal, so that as much RAM as possible is available for the game.
iOS doesn't handle memory in the same way that macOS, Windows, and Linux do. If apps ask for more RAM – and there isn't enough – iOS may go around asking apps to "voluntarily" give up RAM. If apps fail to free enough RAM, iOS will start crashing some deliberately to take memory back anyway. (There is swapping to "compressed RAM", but that only goes so far.)
I'm guessing that Diablo Immortal is a RAM-hungry app, and so closing all of the other apps that may be competing for RAM might reduce the chances of it crashing because of lack of memory.
If the crashes continue when running Diablo Immortal by itself, contact the app vendor (game publisher). It is their responsibility to make their game work on the platforms that they say it supports.