I used to get this bug everytime I loaded an autox layout just after starting a race. I can't remember if it had to be set to a proper race but I remember it happening. I haven't used an autox layout since so it could still be there in V5.
This.. You can drive through them while the layout is loading but I have had it where I am driving around with it loaded with solid walls then they just glitch and become air. I usually get a half a second stutter when they loose solidness