I have a support case with Apple because of this problem, and after doing a bunch of diagnosis and looking at data logs from my iPhone 13 Pro Max, this is what they said:
"Thanks for you patience. I have received an update about your case.
The Apple engineering team has confirmed the retrieval of the log files to further investigate your issue, and at this time we do not have any further steps to take to attempt to resolve the storage issue.
The results of the investigation may come in the form of an update, new product documentation provided by Apple, or a combination of the two. Please keep me up to date if symptoms change, or if you discover a workaround.
I will also be in contact with you as necessary for this issue as I receive updates on my end."
So basically they're admitting there is something wrong with iOS 15 and they need to fix it.