Those settings should be stored and remembered after restarting the aircraft. I did some testing on the current version of DJI GO (iOS) and was unable to reproduce that issue. If you're not running the current version of DJI GO, then maybe try upgrading to the current version.
Does your ISO change when gimble wheel turned(adjustment of contrast),,tried camera reset ?this does ring a bell if it was p3 only having 1 transmission channel it won't go custom but auto is preselecting the best channel for you so has me thinking on this toI uninstalled the app and even reinstalled the drivers for remote and the Phantom drone and still my ISO and white balance would go to auto. No other settings would change .