I have a handful of users connecting their Thunderbird calendars to a list of about eight calendars in Nextcloud 20.0.9. Thunderbird has been configured using TBSync, but historically this has been happening when each calendar is configured separately as well.
As new events are added to the calendar (The user doing this is using the Nextcloud web interface) they are not always being synced with the calendar in Thunderbird. The way that I’m able to force this sync is by deselecting the calendar in TBSync and then selecting it again and manually resysncing.
I’m not sure if this is a problem with Thunderbird specifically or something the Nextcloud is doing that is against what Thunderbird expects. If someone knows the answer directly that is great. I’m open to gathering data and troubleshooting this but I’m not sure where to go from here.