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.
FWIW, I had a similar experience in Seattle on Sunday morning (October 23). Note that I'm a new user of Waze, and this is the first time I'd used it in downtown Seattle. Anyway, the disconnect was on I-5 in both ways between the north edge of downtown and the University district, which is probably the most heavily traveled section of highway in the entire state. Odd that this is where Waze would get lost...
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.