I was under the impression, having been monitoring this site, that the latest updates allowed simultaneous services. I am experiencing just the opposite. In previous versions, the loading/unloading of baggage or cargo always started at the same time as boarding/deboarding was selected in the menu. This was much more realistic. Is this a deliberate change for some reason?
I think you are confusing simultaneous servicing with Auto Mode. They are two entirely separate things.
If you don't use Auto mode, nothing has changed the way boarding/deboarding works and, it's you that will decide if using simultaneous servicing or not, by actually selecting services when others are started, something you couldn't do before.
In this case, in order to prevent vehicle clashing into each other, depending on the order YOU select the services, some of the vehicles will wait for others to arrive or go away.
If you ALSO enable Auto mode, GSX will select the order for you, so it will always use simultaneous servicing.
And please, do not post the same message twice. I removed your other identical message for this reason.