Problems importing photos from SD card

[mebpenguin]

I have had a consistent and very annoying problem importing photos from an SD card to both my iPhone X and original 12.9 iPad Pro. Here’s the situation:

1. I have icloud photo library set up and syncing all of my photos
2. When I look at my device storage in Settings>General it shows that I have half or more of my storage remaining (phone is 256gb, ipad is 128gb)
3. When I try to import a day’s worth of photos, say 10-20gb worth, it says I have insufficient space and to delete apps, even though the device says I should have plenty of space
4. I try restarting the device, this accomplishes nothing
5. I delete stuff to clear space until I can’t delete any further
6. In the end I turn off icloud photo library, nuke the device, restore from a backup, and turn icloud photo library back on, the device works fine for 3-6 months and then the same problem rises up again

I assume there’s some storage being used that isn’t showing up in the settings app, but this is driving me nuts. I use icloud photo library so the device will offload originals so I can keep a decent amount of space free. That this fails on a recurring basis is an obvious bug in the system. The worst thing is that I’ve googled around and have found no one else reporting this.

Anyone else have this problem? Anyone found a solution?