Having same issue on my iPhone 6s Plus with iOS 13.6. Deleted a bunch of photos and messages and the system storage usage just kept increasing more and more making any deletion pointless. Disabled iCloud and now it’s recommending iCloud to me again to make more space....Hahaha..despite same issue existing with iCloud. This is ridiculous.
Using 18.8 gb of Other usage and 5.59 of System usage out of my 32 gb available. No space available.