No file after sharing?

I have a complex Motion project that I've tried to share twice now. I see the file as it's being created, but when the process completes (I have the background task list opened), the file disappears.

I have no issues with other files, but this one (which is taking about an hour to create) just disappears. My machine is a little underpowered, but I'm exporting to an external thunderbolt SSD with plenty of room left. I don't get any error messages. I've now exported it twice with the same result.