Not sure how physics are calculated with preset vehicle on top of a trailer but this looked weird:
https://steamuserimages-a.akamaihd.net/ugc/866241292748693944/3EAA338204EC4BD773C53ADECCB16C6B8BADD308/

When I switched to that car it fell off from that trailer

PC user (master race! )
g29 main in game controller with xbox360 mounted near it for camera control and crane operations.
g13 gaming keyboard. Other peripherals seen by game but not use at all.
Mouse for menus, advanced mode etc.

Wow ... how did you do that xD ? I love to drive around like that on maps, but that has not yet happen to me.
I guess it may have just glitched out or some. Does this happen everytime for you? Not that i can do anything about it, im just interested.

It was this challenge mission with 2 trucks convoy You can try it for yourself and see the results. My trailer got stuck in funny way. Maybe if it would rotate more then this car would drop on its own but my trailer stuck in that position and did not move further so I noticed that ihis UAZ is holding to the trailer in the weird way so I decided to switch to that UAZ and after that it fell off and drown