I attempted to board a BA flight at ZRH today (flying ZRH->LHR->SFO). I had checked in electronically and been issued with a mobile boarding pass. At the gate the machine beeped and I got sent to a supervisor (DNATA I presume) who asked for my "printed ESTA". I calmly explained I didn't have a printed ESTA, Timatic does not require a printed ESTA (*), I had never previously been asked for a printed ESTA despite having flown this route many tens of times, and BA's own systems check ESTA validity. I was told that "we need to check has your ESTA might have expired" (I did not explain that a printed ESTA might have been cancelled and it is trivial to alter a printed 'application success' PDF to a later expiry date). After a fair amount of to and fro (as I didn't have the ESTA application handy on my phone) they said "let me check the system" and after a few minutes let me through.

Is there some new requirement to have a printed ESTA handy? I thought the whole point about the ESTA was that it is electronic (the clue being in the first initial). Or is this some fantastic new BA IT innovation? Or have I just been lucky many tens of times (I have Global Entry if that's relevant)?

* = I think there is technically no such thing as a printed ESTA. You can merely print the application status page that says "successful" - the ESTA itself is not a (publicly accessible) document.