Sigh… I had hoped that ST finally fixed the scheduling problem. Obviously not. In the last 3 days/nights my (long-running) smartapp failed to reschedule once each day, hence, stops working. No specific time when it failed. Nothing in the log that indicated a problem.
When working properly the app should run once on the hour then schedule the next run 1 hour later. For unknown reasons, in past 3 days, this rescheduling has failed once each day.