The export progress dialog says "Keep Final Cut Pro open until the export is complete", and the standard iPadOS limitations are that background tasks are killed after either 10 minutes or when some foreground app wants more RAM. So it it's not instantly cancelled but it's a precarious workflow compared to on a Mac.

Can neither confirm nor deny :) I've seen people complain about this on Twitter and Mastodon though.

It's possible people are running into iOS limitations: it will kill apps when it thinks there's not enough memory.