The truck will not disappear, but it cannot be switched or activated.Until you put it on another trailer, then unpinned
Dido. It's really fun when it happens in the wild.
A similar bug happens if you pack a truck on a flatbed or sideboard addon of another truck and then enter the garage with that truck. The packed truck will then be stuck in the air, unable to be selected.
you don't need to put it back on trailer fully - just front wheels on a ramp is enough. but it's wierd.