How can I stop my iPhone from “Cleaning” apps in the background? As of a few months ago I began to notice my iPhone would automatically close and unload open programs if I don’t have them immediately running. This effectively eliminates any “multitasking” ability the iPhone used to have. For example, if I have a spreadsheet open and I switch to email, after a few seconds in the background the spreadsheet app will automatically close; if I reload it then I need to go through the process of opening the app, reopening the file, and waiting for it to load (which can take several minutes with a complex file). I’ll be on a social site, see a thread that I’d like to comment on by linking an article, minimize that app, find the article in Safari, and return to my app, only to find that the social app was closed for me and I probably can’t find the thread again. Playing a game? If the game doesn’t constantly auto save then I lose all progress if I answer a phone call while playing. 

I’ve managed to catch the iPhone “Cleaning” my apps if I watch the main screen - the opened app will grey out for a second and the word “Cleaning” will appear, after which I find the app has been closed and I’ve lost my data / place / progress. So it seems to me that preventing this from happening is the answer. But I can’t seem to disable it. Any ideas? (Before someone asks the obvious, I have 3GB of space open and unused on the phone. Also to be clear, this has nothing to do with the “Offload Unused Apps” feature.)
You can't. It's up to the individual app to save the current context if the user switches to another app. Some apps handle this better than others, but in any case this issue needs to be addressed to the developers of the apps.