I had this happen on my Prius a couple of years ago.
One reason it can happen is if you are in a satellite "black-spot". If it happens in the same place all the time then this may well be the reason.
The other possibility is that there is a fault with the unit - as was the case with my Prius. After much checking in my case Toyota replaced the whole unit at several thousand pounds (apparently).
If it is not caused by the first option then I would speak with your dealership.
One reason it can happen is if you are in a satellite "black-spot". If it happens in the same place all the time then this may well be the reason.
The other possibility is that there is a fault with the unit - as was the case with my Prius. After much checking in my case Toyota replaced the whole unit at several thousand pounds (apparently).
If it is not caused by the first option then I would speak with your dealership.