Well, curiously I encountered a similar issue yesterday while checking in for a flight with United (UA).

My M+B profile does not have the extra given or middle names that are common in Belgium. In the past this was never an issue whlle flying with SK and United on bookings that did include those names.

About 6 months ago I made a booking for a flight that I am on right now. It included my A3 number. It was accepted without issues and showed the extra luggage and gold status. But yesterday when checked in, my FFP number was gone. The UA website claimed that 'the information did not correspond to their records' when I tried to enter it. I then called United using the Belgian phone number and the agent confirmed that it was a name error. He could not change it either.

This morning when dropping off my bag the super friendly agent at BRU also tried to do it, all to no avail. She confirmed that their system enforces this. The end result was no priority tags on the luggage and no fast lane. I did not mind this at all because security was almost empty and I can access the lounge with PP or my M+B card. But I will need to ask for a retro-credit from Aegean.

I had some time to kill at the gate, so I approached an agent that was available and asked what I did wrong and if I needed to enter these names in a different way. (I seem to remember that I put the in the 'first name' field instead of the middle name.) But he also confirmed that the FFP-card and booking need to be exactly the same. But, assumed it should be possible to travel with only the first name.

Other frequent flyers I know also confirm using only their first name and omitting the middle names in all international/intercontinental bookings and not encountering any issues when crossing borders. One of them did mention that he had the same FFP error on a recent TK flight. As multiple carriers are using similar or the same IT-systems in the backend, this could be more widespread.

To be honest, I am a bit wary of using only the first name for flights entering the USA, as I imagine it could mess with the CBP or DHS border processes. But I also do not look forward to messing with my A3-account. The Aegean agents are very helpful and knowledgeable, but I would hate to complicate things.

Maybe there are Star Alliance or Aegean lurkers here that could shed some light on the matter? So I concur with the posters above: make sure the info is identical.

Edit: typo