I am guessing you're on ios 15?
I recall a bug in 15.0 that was causing storage enumeration issues:
https://www.macrumors.com/2021/09/22...st-full-alert/
That link talks about both 'almost full' and showing the increasing empty size that you're seeing
Seems to be unresolved (or at least not completely resolved yet) - perhaps in 15.1?