Same problem here, and just got off chatting with the useless Apple chat support executives. Idiots wasted my time and just suggested to do factory reset followed by backup!! On my iphone, it happened because I was trying to send large files by maildrop. I was trying to send 500+ pics/videos, having size of about 3.5 GB using email/mail drop. I tried 3-4 times, and Every time the process failed it kept keeping that much space blocked!! So after 3 attempts I got left with 0 space and about 13 GB of data showing in “Other System Data”! And that, after I deleted so many apps, files, to make way for more space!! Basically, Apple keeps storing the deleted file in cache, and doesn’t even allow us to delete those!! Btw, I already tried the usual suggestions, and they all fail- clearing safari cache, disconnecting mail accounts, etc etc. Nothing works, and I don’t want to go for Facroey reset. What a ****** situation!