I’m using an iPhone 11 Pro on iOS 14, but I have also observed this behavior on iOS 13 as well as on an iPad running iOS 14.
Sometimes, when I go to reboot my phone I will close all apps (just to cleanup the app drawer, I’m not trying to manage what’s running myself) and then put the phone into airplane mode so that it will be in airplane mode when it eventually reboots.
What I’ve noticed is that about half the time, after closing all apps and putting the phone into airplane mode, when the phone reboots, LTE is active, wifi is not connected, and the last few apps that I closed are in the app switcher. If I do it again, it works as expected. I also notice that if I wait for 30-60 seconds after closing everything, and then reboot, I almost never see this behavior.
This is more of a curiosity than anything but I wanted to see if there was a reason for this? Does iOS not immediately store the open apps and airplane mode status when set?