^^^^^^TL;DR:
I frequently film hour long videos on my phone and keep a lot of space free on my phone so that I can do this when I need to. I waited a long time to upgrade to iOS 11.4 from iOS 10. After upgrading, filming an hour long video, and deleting it, my "System Storage" went from 4-5 gigs to 23 gigs. Surely when I delete things permanently, the new iOS is putting it in the System Storage.
Please fix this bug 😟