Probably this is the wrong place to post this, but for what it's worth, I had the same issue twice last week whilst travelling along familiar routes in the Hounslow/Feltham area. Whilst glancing at the map, I noticed that the purple route just stopped, but as I continued to drive I could see that further up ahead the purple route started again.
I thought that it was an app related issue, until it also happened on the Livemap last week whilst planning a route between Sunbury-on-Thames & Kingston-upon-Thames. Of the three routes recommended, there was a very obvious gap in the middle of the purple route. Today, it's fine.