Sometimes when I hop out of a vehicle I find that my ruck has been restored.
Most times I'm not that lucky.
Once in a while I see "ruck" in the vehicles gear. When I unload it I expect to see it on the ground ... but I don't know, because I can't recall ever having this work.
When you mean "unload it" how did you perform that action?
Was it via "Gear" menu, or ACE interaction keys?
The basics of any "seats" that you cannot or should not occupy unless you're a "Crew" or "Pilot" are: Crew Chief seats, Gunner seats, rear gunner seats, pilot seat, co-pilot seats, Driver seats, loader seats, commander seats, etc. Basically, if you are not supposed to be a pilot or a crewman of certain vehicles. Don't.
Certain missions, will allow you to retrieve your gear. Some will not. Call it a mission design, or ACE update glitch. It just happens.
There are two ACE interaction keys. The SELF interaction, and the WORLD interaction (opposite of self).
Use the World interaction to retrieve your ruck off a vehicle.