OK, Stopping Sync and restarting brought back the Sync icon for Collections.
Although new Collection Sets are not syncing. (Also, why are sets called Albums on a mobile device?) Anyway, new Sets are not syncing, but their contents are. And now if I create a new "album" on a mobile deive, it does not appear in Classic as a Set.
I've used LR for syncing for a while now, and I find in general the whole process is kind of janky. Sometimes it works as it should, and sometimes I have to quit this or that or make open things in a certian sequence to get syncing to actually take place. Which now doesn't seem to be happening. Sync is a great feature, wish it worked smoothly and predictably.