OK the tldr; on this is update is: I’m an idiot
You can stop reading here if you don’t need further info 
Back on the 18th I updated my error catching code in a way that also caught expired token issues - I usually handle those by refreshing the token & carrying on with the sync, but my update handled those as if they were Strava errors. I handle those by scheduling them in the next API cutoff (a long-ish term try again). 
This means everyone that’s missing an activity (these tens of thousands of activities) are just circling the queue → attempting to sync → getting an expired token error → re-added back to the end of the queue. 
I’m releasing a fix which reverts to the correct behavior. I’ll keep the temporary history sync paused while the 12k new activities get synced into CityStrides. I’m expecting this to take 2-4 hours. I’ll check back later today, and if those activities have synced in I’ll allow historic syncing again.
sorry 
Update: The queue has been completely worked through, so there shouldn’t be any missing Strava activities from the 18th → today