I've also seen the "Proceed to highlighted route" message with constant recalculation when the first segment of your route is a long segment - it keeps trying to recalculate your route until you get to your route's second segment, and if getting to the second segment requires a turn, it doesn't get announced. I also noticed this happens every time you start on a longer segment. It probably happens on every first segment but is more noticeable when the first segment is longer.
It's a routing server problem sending bad data to the client for the route which causes the client to think it's offroute, or the periodic check if there is a better route is coming back in an unexpected way making the client think it needs to recalc the route.
-- If it looks strange, or I used bad grammar or misspelled anything, it's my phone's fault.
It happens to me from time to time. More often than not, however, if is appropriately when I am not on the route yet. But I have had several occasions where I was on the route and my car showed me on the route. One time for nearly a km.