Auto Hold Question

[Stroy]

Hey all,

I tried searching but couldn't find an answer. I've noticed every now and then that the auto hold feature will change over to the actual park(red) mode when I'm stopped at a traffic light. It really startles me because I'll activate auto hold when I'm stuck in NYC traffic so I don't have to keep my foot on the brake constantly. Then, when I step on the gas when the traffic light turns green, I get the warning bell and the red parking brake indicator goes on and the truck won't move. It only happens on occasion. Does this sound normal? In the manual, it says that the auto hold feature is deactivated automatically if the vehicle is braked to a stop. That doesn't make sense though because I'll leave auto hold on and stop the truck and it usually stays engaged. Sorry for the long winded post. Thanks.