Just to follow up on this SSSS'd trip LHR-EWR-BOS:
(1) "SSSS" on boarding pass at LHR meant that after presenting boarding pass, I was escorted to a room next to the gate area where a person swabbed all my electronics, stuck the swab in a machine, waited for a beep, put a sticker on my boarding pass, and let me go.
(2) At EWR, *was* able to re-print boarding pass EWR-BOS at a kiosk or via mobile app. It did *not* say PreCheck.
(3) OLCI'd for another trip today and boarding pass reflects Pre-Check as usual.
So as UA and others here suggested, maybe a random one-off.