Memory usage fault

Recently purchased an iPad Pro 128GB, having memory usage issue when using my GoPro, so I have for example 100GB showing in system available, I have say 20GB on GoPro, I download straight to photos, then I use iMovie to edit all these clips and create a Movie, so now about 15 GB which I now save back to photos ok!, now I delete all my original clips from photos, so far so good, now if I check memory it’s quite full but after a while this iMovie is uploaded to iCloud ok, I can now watch on iPhone ok!

So if I leave overnight this now clears space on my hard drive so I can be showing 100 GB available again, now I could do this procedure once more then that’s it, iPad will say it’s full even if I’m showing 100GB available ?, the only way I can sort it out is to full restore!! But what I have noticed if I go into files even though I’m showing say 100 GB available in system it will say for example 7 GB available, only way to resolve this is restore, and these nothing in files, any help please, my iPad is two months old and had six full restores and three phone calls to Apple who are blaming the GoPro!!!, I just can’t understand why the memory in system can say 100 GB and in files say 7 GB for example then not allow me to import any more, any help please !,