For lazy people who dont want to click in the link in my above post, this is what nikka wrote:
When a player is in pit (PLP), his MCI CompCar struct is set to 0, including Info byte.
So, if his CompCar is CCI_FIRST or CCI_LAST, this info is missing.
(yes, this one caused some brain twisting... )
Edit: Not entire struct is set to 0, PLID is still intact.