TL:DR - PLEASE fix the vehicle cancel options on trailers and towed vehicles. If one or the other gets cancelled by the player, automatically cancel the other unit it is attached to.
I had a mission with multiple patients and send a EMS Mass Casualty Trailer (large) to. (It attached to the semi tug.)
I accidentally clicked cancel on either the trailer or the tug (I think the trailer.)
I transported all patients and the mission is closed. (Fire Truck Accident)
1x EMS Mobile Command Unit and 1x EMS Operations Support are also stuck on this mission.
The tug is showing the mission has a "Transport needed" status but I cannot get into the mission to do anything. (lightning bolt server error)
As I'm chugging through other ems transports to hospitals, a screen pops up asking what vehicle to tow the trailer with.
I tried dispatching these units to another call, but the mission list is now gone from the vehicle menu.
WORKAROUND FIX (This is NOT a solution to the problem, just a backwards way to get out of it. Please address this bug.)
I was able to go into the "next mission" that I assigned the vehicles to and "send directly to mission" to get them to appear in that mission and get them back.