I haven't noticed it. but it's probably normal. The clock will run for a bit after shutting off, then stop. When you start the truck, it may take a few to update.
I say this as my Genesis with a analog clock was similar. If it was shut off for a while, when I'd go and restart it, the hands on the clock would spin around until it got to the correct time.
So my thinking is, the truck is doing the same. only update the clock when the truck is running.