As far as I know this is not really a bug. Just an oversight on transporting. The game works the following both Freight and Passengers:
Can i get from here to here within 20 Minutes (~ 1 – 2 ingame years.) And will it take me where i want to go (Source -> Destination. EX: Home to work. Freight supply to demand)
If the game thinks it can it will start its trip. If it knows it cant it wont. The frequency of cargo travel however seems finicky. It shows the wait time from vehicle to another vehicle per station averages. But it dosnt show the true travel time from point A to B and there is no real way to determine how long it truelly takes for a ‘agent’ (freight, passenger) to get to where they want to go other then simply guessing.
So bassically i believe that the game will send out an agent to do their thing. Even tho it calculates the time to get from point A to B less then 20 minutes, It doesnt count in the travel time. If vehicle spacing is bad (train 1 after another and then a huge wait for another train) it still will send out the agent and the agent dissapears later on since the travel time is too long. This includes at stations, and hilariouslly in transit. I thought of this as they simply die out in transit but its really the game just pulling them back since it took too long. Its not a bug. Just abit weird.
Id say there should atleast be alittle compensation. Like half the usual income got from the dissapearing agent and the agent occupies the actual vehicle until a station to disembark.