Backup completes long after curl exits

When I launch a BackWPup job from a remote host using a script, like this:

curl "https://example.com/wp-cron.php?_nonce=xxxxxxxx&backwpup_run=runext&jobid=1"

I’ve noticed that when curl exits, the backup file (say, /var/backups/mybackup.tar.gz) isn’t completely written yet. It takes another minute or so for the job to finish writing the backup file. This is a problem because my script tries to copy the backup file to the remote host immediately after curl exits (for example, running scp example.com:/var/backups/mybackup.tar.gz .), but the backup file is incomplete at that time.

Is there a way to change this behavior so the backup runs synchronously, so curl exits after the backup file is completely written?

Thank you very much.

System details: BackWPup version 4.0.1, Version 6.4.1, PHP 8.1.2, hosted on Ubuntu Linux 22.04 LTS.