Rated 3 out of 5 stars

Same problem as TeeBee (April 25, 2019) - the contacts got synchronized upon initial account add. But never since. There is also no option to manually sync the addressbooks.
Calendar is working on the other hand.
Using nextcloud as backend.

Same answer: Contact me directly. Either by sending in a bug report under help in the tbsync manager or by using my public email address listed here:
https://addons.thunderbird.net/de/thunderbird/addon/tbsync/

I can only guess, that you did not enable auto sync...