iOS Disregards Airplane Mode Setting on Reboot
This is by design.
I've tested this on several iOS devices of various iOS versions (iOS 9, 13, 14 and iPadOS 14) and all do the same thing.
I haven't found anything that specifically says so, but it makes sense so that if it's ever lost/stolen while in Airplane mode (WiFi/Bluetooth/Cellular off) when it comes back on after a period of time, it will automatically turn things on to be able to be found.
If you turn off WiFi individually, you get a message that it's &quot;disconnecting from WiFi for 24 hours,&quot; not permanently. This is obviously to get you connected again in case you forget or the phone is lost or stolen.
It falls into a design philosophy of &quot;fail secure&quot; meaning if everything fails, it will fall back to a config that will allows you to find it and/or wipe it.