OK. At least with my iPhone I have found the solution.
(16GB 5S with 10.2, but the problems really started with upgrading to 10).
As you, I did complete recovery and was temporarily happy, to see after few hours diminishing free memory. I have seen for the first time in my life "zero memory" in my iPhone. Still if you added size of the storage taken by apps and their files in Settings/General/Disk management, my ading result was 2,6GB, which is obviously far below 16 GB. ***??? Going through support pages and forums actually gave me a hint to look for Apple applications, which I have previously deleted. The hint was, that they are actually not deleted, but hidden. I have reviewed few and finally found it! In my case, the solution was to reinstall (or unhide) the Podcast application. After I did it I saw tones of podcasts downloaded and waiting to be downloaded. It looks like "deleted" Podcast app actually tried to download podcasts whenever there was any free memory available :-). I have deleted all podcasts including links - this is important - because they are little zombies. It was difficult with one, Woodsongs videopodcast (btw great!) returning all the time. I do not know why. Finally I gave up and have changed setup of this podcast to download only one episode and remove it after viewing. Works, but there is obviously a bug in podcasts app. I hope support people read this.
I hope this can help some of you :-)
Good luck in fighting with Apple zombie bug.