Here are some posts on the subject, including some information from the developer of the feature:
https://forums.truenas.com/t/how-do-i-troubleshoot-no-automatic-time-machine-snapshots/7856
https://forums.truenas.com/t/cloud-sync-time-machine-sparsebunde-to-b2/4962/2
https://www.truenas.com/community/threads/timemachine-snapshots-retention.100593/