Middle name not entered in the booking should be no problem. I and others have flown without middle names many times in the past.

I have a different situation. Family trip and one person told me their passport middle name as they recalled it which turns out to be wrong. They thought they had two middle names on the passport but really it is only one and I needed to book to grab award space before they could confirm with the physical passport. So now they have:

Actual Name: First Middle1 Last
Booked Name: First Middle1Middle2 Last

So, first and last name matches, first name in middle field matches but there is an extra middle name in the booking that is not on the passport. In other words, it is not just a middle name that was not entered but a middle name that was entered but does not match. At least the first part of it matches. This is a partner award booking and for the longest time there was no way to change it since the airline would insist on a new booking and there was no award space. Now there is award space but rather than pay a large change fee I have decided to wing it at airport check-in. I have searched extensively and outside of Vietnam it is almost unheard of for someone to be rejected for a middle name mismatch so I am going to risk it. If they question it I will point out that our full passport information has been entered into the booking for months and if there was a problem with a mismatch they should have contacted us in advance.

If anyone has experience with a similar situation please advise how it turned out. If this were not a partner booking I would expect it could easily be fixed by the airport agent fixing the name. On a partner booking I am not as confident but still willing to risk it. My backup plan is to book a last minute one-way ticket for one person using either miles or money, possibly on a different carrier and class of service.