Very, over simplified, part of the problem is that Apple mobile software likes to shut down apps that have a spike in RAM use. So when the game is trying to switch maps in a dialogue, or with an quest update (say with an rescued NPC in tow), that can cross that invisible line. It's a memory management issue that is not unique to Apple, but Apple prioritization of other Apple products tends to make third party apps the first to force close (i.e. crash), even if the app is running normally.
Closing apps running the background, turning off app permissions to monitor everything you do, and only running minimal apps can all help as well.
A lot of Apple official apps have prioritization for RAM. Apple will shut down your heartbeat before it closes it's consumer information gathering apps.