iPhone claims upwards of 80GB are free, but storage is actually full.

iPhone 11 Pro with iOS 17.5

According to iTunes and my iPhone's Settings, which don't agree, I have between 80-90GB of free space available of 256GB. When I attempt to move a video onto my iPhone through iTunes, it says that the move failed because my storage is full.

The app Unveil, which is no longer on the App Store anymore for some reason, seemingly correctly reports my storage as 1GB remaining.

From this point on in my question I'll rely on Unveil, since it's the only accurate free-space-reporter.

Deleting a 2GB file from Files and Recently Deleted showed no change in my available storage. A restart did not correct the error. I checked Unveil a few hours later, and my free storage had gone up by 1GB. A few hours later, it had gone up by 4GB. A few hours later, it was down to less than 1GB free. I did not download, add, or delete anything on my iPhone during this period, but attempting to move videos onto my iPhone through iTunes was met with appropriate responses according to Unveil's free space claims.

Over the last few weeks, my true available storage has slowly shrunk, as iTunes and Settings have reported more and more available. Two weeks ago, I had 70GB free in Settings/iTunes, and could not move anything onto my iPhone.

It almost seems like dice are being rolled constantly to decide how much space I actually have free; the space range is also shrinking. iTunes and Settings are clearly not reporting that space accurately, as well. Adding up all of my apps and system data shown in Settings, along with the size of my folders in Files, comes up with 90GB free. So where are those invisible gigabytes being used?

From what I can find on these forums, this type of free-space-reporting problem has been reported and widespread concurred with since 2018. Why has it not been fixed? It's becoming a serious issue for me for my iPhone's storage to shrink.