Departing passengers do not enter the terminal. I never see passengers in queue through Security. Nobody waits at the gate, waiting to board. Checking the Stand info popup indicates Passenger Loading Complete. Yet, it shows no passengers boarded.
Vers 24.3-2 did not have the first problem. Passengers entered the terminal, went through Security, and waited at gates. In fact, I would watch the passengers enter the aircraft. It still had the problem showing 0 passengers on the Stand info popup.
And, of course, as soon as I created this thread, it started working. The queue to the Medium Security gate was stuck. It had both passengers and employees in it. I cleared the Security queue by deactivating and reactivating it. Also built an employee-only small security entry.
After doing the above, that’s when no passengers ever showed up, as I described in the OP. I saved and reloaded the game. After awhile, passengers finally arrived at the medium security checkpoint and proceeded to the gate, where they sat in the available seating. Finally, at boarding time, they all boarded the aircraft, and the stand info popup showed the correct number had boarded.
I’ll just chalk this up to being alpha, and some unexplainable glitches will occur. No problemo.
EDIT: I’m still seeing a lot of O passengers loaded in the stand info popup, yet it indicates Boarding Completed.
EDIT 2: opened bug report ACEO-3322. Olof responded it is a known bug and devs are collecting save games to determine the cause.