Just found this thread.I've certainly found so far that registering in one country doesn't seem to mean the next Schengen country will recognise it yet. I've registered multiple times this year. Twice at FCO where everyone, despite the fact I had a green EES sticker on my passport with FCO on it, was being made to use the booths.

Then the next time I went to FCO the booths weren't working and the all passports queue was horrific. One of the staff directing people to queues clocked my FCO EES sticker and sent me to a small bank of completely empty egates which could only be used if you'd previously registered in Italy (her words, not mine, so whether having registered in another Schengen location would have still worked I don't know). So I was saved on that occasion.

All in all an absolute shambles and anyone heading to any Schengen destination in the Easter holidays where UK passports can't use egates as standards are going to have an absolutely horrendous wait.