Could be Missing queued Strava activities still missing - #33 by JamesChevalier if it just started happening on the 18th … the activity from this morning isn’t from that issue, though.
I had another person report “group runs” not syncing, but they did eventually come through without me doing anything - I’m wondering if Strava’s alert system is delayed…
Next time you notice this, if you have the patience try & wait (some hours?) to see if it appears. Let me know if/when that happens.