iMovie export claims not enough space to export when there ought to be plenty

In iMovie on an iPad Pro (4th generation with 512 GB) and IOS 17.5.1, trying to export a 7-minute 4K movie to Photos, I get an error message, "there is not enough space to export this movie". But I should have plenty:

• iPad storage says 86.19 GB of 512 GB used (that is, more than 80% of the device is free)
• iCloud storage says 939.4 GB of 2 TB used (that is, just over 1 TB should be available)

Rebooting, offloading apps, and deleting pretty much all iCloud downloads eventually got me enough room, but there seems to be ample storage, both on the device and in the cloud. The (eventually) exported video was a "mere" 1.4 GB in size.

Any ideas? Could the OS be woefully underreporting my device's usage? Or could exporting be creating a truly massive temp file? You'll note I even splurged on the 512 GB model to avoid problems like this, so this is pretty frustrating.