iPhone keeps saying it's out of memory...but it's not

I was using my iPhone 6S 64GB this morning to record a long video (two hours long).

Before beginning, I made sure I had plenty of extra space (more than 10GB). See attached screenshots. But throughout the recording, I would get persistent error messages that I was out of space. I went in and deleted some less-important videos and apps, then continued recording, but still got the errors after a few more minutes.

The iPhone would also hang a very long time while retrieving the info for the Settings > General > iPhone storage page.

I ran a few tests and was temporarily able to get it to record some more video, but it would always eventually give me the out of storage space error.

How can I diagnose and fix this problem -- at least -- get an accurate count of how much space is free on my phone, and then be able to record up to the limit of its storage? Right now, it's not doing that.

Thanks!