Running Firefox 82.0 on three computers on which I utilise Firefox Sync.
The actual synchronisation works OK, but I recently noticed (under Firefox Account -> Account Settings -> Devices & apps) the "Last seen" date/time is no longer being updated, as shown in this screenshot:
https://paste.opensuse.org/view/raw/44c1cc59
All three machine shown there were synchronised (at least once) today.
Bug, or a "removed feature"?
Edit: Should add, using "Refresh" doesn't change the last seen date.