After 3 full resets with the same issue each time (growing system data from 6gb up to 92g) I finally worked out that it was an issue with iCloud Photos. I always have it set to ‘optimise iPhone storage’ so I clicked download and keep originals and then back on optimise iPhone storage. And the issue has just disappeared. It seems the phone was holding on to 10 years of photos and videos and not firing them off to iCloud automatically as it usually does. Only downloading old photos etc as I require.
A bit of a headache for a week, waking up to iPhone storage full every morning.
A bit of a headache for a week, waking up to iPhone storage full every morning.