Update: Mrs drpabs checked and her flight yesterday JER-LHR (on which she was seated beside me) is showing as not flown. Thus the problem yesterday definitely lies with JER not correctly recording the scanning of our boarding passes at the gate, as kindly explained by KARFA and Ldnn1 above. As a coda, I have been able to successfully check in for the next flight in my itinerary.

Thanks for the swift replies and I can now relax and have a nice sunny day in Bucharest (with a brief work meeting at this afternoon) including a walk in the excellent Cișmigiu Park.