I have the same storage problem on an Iphone 11.
I have about 80GB free according to the IOS, but the Blackmagic app shows I have 6GB.
I wonder if this is related., the size I'm getting on the Blackmagic app seems to correspond with the free space before I added Optimized Storage for my photos.
https://support.apple.com/en-au/HT205703It will be interesting to hear if anyone else has enabled "Iphone optimization" on the photo settings and also has this problem.
This is what apple has on their site about optimization.
When Optimise Storage is turned on, full-resolution photos and videos are stored in iCloud, and, when needed, space-saving copies are stored on your device. And as long as you have enough space in iCloud, you can store as many photos and videos as you want.