I'm flying a Mavic 2 Pro using DJI Go 4 to control it from an iPad Mini 2 running IOS 12.5.6  The iPad does NOT have a sim card so only works on wifi.

Nothing has changed with any of the software or hardware for over a year but about a month ago the app froze in mid flight and then the screen went dark. It then returned to the home screen and I re-launched Go4. The drone had stayed where it was and everything resumed. More recently though, it has happened on every flight and today it happened three times in one flight.

Anyone got any idea what's going on ?

Thank you

Since it had worked for you previously, just got to ask, how long has it been since you have restarted the iPad?  Hate to give a help-desk like answer, but these tablets don't have a lot of system memory and need a restart once in a while.

I too would ask how much free memory does the iPad have and can you clear some 'junk' out?

The other standard answer is the good ole delete, restart the ipad, then download again.
Also, isn't the iPad Mini 2 a really old tablet?

Hello there Nevtan. I am sorry for the trouble this has caused and thank you for reaching out. Kindly please try to uninstall and reinstall the DJI Go 4 ( https://www.dji.com/downloads/djiapp/dji-go-4 ) application to see if the issue will still persist? In addition, please sync all of your flight records before uninstalling the DJI Go 4 application on your mobile device for you not to loose your previous flight information. If the issue persist kindly please try using a different recommended mobile device for us to be able to isolate the said issue. Thank you.

This behaviour can also be caused by a dodgy cable connection. I see it quite often with an Android tablet.
Getting decent cables has been an ongoing problem, some cables do not even work when brand new! The one I'm currently using causes DJI GO4 to freeze, with a message about losing contact once or twice every few flights. Sometimes at random, and sometimes when the cable is touched.