O.k. - took my time and took a closer look:
The flight in question is SL314. It will depart at 16:00. I followed my PAX closely this time. They arrived about 1h before the flight even landed on my RWY. Check-in was at 14:00 or so and went smoothly for all 18 PAX - done in 10 minutes max. They were waiting in the hall until 14:57 when the boarding desk opened in time.
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They reached the plane and boarded regularly until 14/18 were aboard. Then suddenly, boarding ended and the 4 PAX highlighted were left at 15:32, so 28 minutes till departure.
I have a suspicion about this. I am playing with 100% passenger per flight ratio while my old airports used 50% ratio. Could it be, that internally the plane is ‘full’ as under 50% rules so surplus PAX arrive at airport but are sent away. Something like under 100% PPF ratio a DC6 will unload 18-22 passengers and has a likewise capacity in its description, but will internally accept only enough passengers as if under 50% PPF ratio which would be about 10-14? I do not know, how the game checks the planes for being full, but let us for simplicity’s sake assume, that it just counts the PAX boarding and compares it with a number in some data file for the plane. While the flight description reads that 18 persons will board, the internal number was not changed and is a 14. So while the game correctly spawns the 100% number of PAX, the plane only will accept the reduced numbers as the plane is full too soon. It is like the PPF ratio affects the number of PAX spawned by planes and world, but the instance that checks whether all PAX are aboard uses another number different from what we see.
Filed in as bug report ACEO-14695