storage – iMovie doesn’t appear to be able to use all the available space on iPad

When trying to save a 15-minute movie from iMovie in order to upload it in the YouTube app, I frequently get the message “Storage Full: There is not enough space left on this device. You cannot share until you free some space.”

This despite the fact that Settings shows “28.8 GB of 64 GB Used” and that iMovie is using only 1.29 GB of that. (It also shows that iMovie was “Last Used: 12/12/18” which is clearly false; is that a clue that the storage graph is inaccurate?)

How can I get iMovie to use this 35G available space for its export?

iPad Air 2, iPadOS 14.2, iMovie 2.3.2 (5087)