Backup completed with warning has some comments, and points to the pull request which is fixing this.
I don’t think the warning itself is a concern, but interrupted backups might be. There are fixes there too.
Until current Canary fixes reach Beta release (assume you’re on 2.0.5.1), avoid stopping Duplicati hard.
Safest current plan is probably “Stop after current file” then wait for work-in-progress to finish uploading.