In the Old Man scenario, if you load a Quadbike into a Van (cargo) (vehicle-in-vehicle feature) and then take the van for a drive, the quadbike in cargo will disappear just after passing a checkpoint.
This bug makes the vehicle-in-vehicle feature of the Van (cargo) useless which hurts gameplay.
I have tested and confirmed that this bug occurs for both the CSAT checkpoint south of Luganville and the Gendarmerie checkpoint north of Luganville.
Below is the saved game file taken just before the quadbike disappears from cargo. From that save point, I pass through the CSAT checkpoint and just after passing the CSAT checkpoint, the quadbike disappears from cargo.