For the past update or 2, whenever I switch to text or something, when I go back to the game it restarts. This a huge pain as I try to complete the 4 waves Deadpool stage. I keep starting over. Please fix the app to stay persistent in the ram.
Yeah I’ve had similar issues and mine will even crash mid-fight. Black screen o death that leads either to my phone rebooting itself or I gotta do a battery pull. It’s beyond frustrating. 
Mine does it sometimes just starting a match. Past 2 events where I found a match that would put me at 1k points I press fight and it resets. Thus causing me to lose points and not get the cover. Also wasting my HP because I need to reshield asap to keep placement that I DROP to. Really annoying.
I think I have read this on the forums in the past, it’s your phone/device, not the app. When your phone/device is short on memory, it chooses which apps to prioritize, so if you’re switching back and forth, you might go back and it restarts. I have noticed it more in the more recent updates, but there is a lot more stuff going on in the game now, than in the past. Don’t think you’ll see any fix. Just try not to switch apps, answer calls, read/respond to texts, or you risk the game starting over.
iOS gives apps 3 minutes to finish their business while in the background, then deems them a potential target for killing when the device runs low on memory. If you want to switch between apps, don’t take longer than 3 minutes. 
My ipad doesn’t do it, but if I let the phone go to standby (screen goes black), sometimes for even a few seconds, it will restart and I lose the game. Happened to me on the DDQ final node yesterday. I was a little mad.
I wish I could figure out what “background” service or auto updating is chewing through the memory so fast that it does this. I thought I had it all turned off. 