Middle name missing - Finnair

I know this has been answered a million times. I researched it extensively when booking, and here we are.

Booked an international ticket through BA, operated by Finnair. I’m a frequent flyer on BA and BA bookings only use first and last name.

Go to check in online through the Finnair website for my flight tomorrow. App scans my passport. No match because of my middle name.

No problem. Gave Finnair a call to see if I could avoid having to check in at the airport tomorrow. The agent told me they will not let me fly.

Is he mistaken?? I have absolutely never heard of this being a policy. The Finnair website says multiple first names need not be included, but this is a little different. Not sure if that’s a language barrier and multiple first names mean first and middle.