I think this is related to some unexpected behavior we observed a few weeks back where the lid of a closed dumpster was considered to be darkness. I think there is a fundamental conceptual confusion within the I7 standard rules regarding components: the stated intention is that they are on the exterior of the parent (and thus visible from outside), but the code tends to treat them as being identical to the parent. That is, if I have a thing T with a component C, the standard rules tend to treat things contained by C as if they were contained by T. This causes obvious problems if you have something like a desk (supporter) that incorporates a drawer (container), so the rules have special cases that make things work most of the time, but these break down once you move beyond these simple cases.
In your case, the player is contained by the tailgate, which is part of the truck, which is contained by the room. When you leave the tailgate, you enter (the core of) the parent of the core of the tailgate. The core of the tailgate is the truck, the parent of the truck is the room, and the room is its own core, so you end up in the room.
But what if you put something inside the tailgate, such as a rug or a box, and stand on/inside that. Now, when you exit, you would go to the core of the parent of (the core of) the rug. The rug is its own core, its parent is the tailgate, and the tailgate’s the core is the truck, so exiting the rug teleports you inside the truck!