I heard of an instance (maybe more than one) when there have been similar issues on a plane with a cell phone. And it wasn’t being charged. It just blew up.
There’s a convenience store security video on YouTube that shows a phone in a guys pants pocket just spontaneously combusting.
As I understand it, phones lighting off isn’t all that rare.
My wife has a carry one bag with a built in battery pack. It is so you can charge your phone off the battery. We gate checked it on a recent trip and the first thing the gate agent checked was IF we had taken that battery out of the luggage.
The last thing they want is one of those batteries catching fire down in the cargo hold. It could literally bring the plane down.