Been seeing similar behaviour for the past few weeks when planning a new route with no intermediate stops. These screengrabs were taken earlier today:
On the initial screen, the mileage is completely wrong - it's neither the total route distance nor the distance to the next junction of interest. After accepting the route, the correct mileage is then shown at the bottom of the screen.
However, in both cases, the ETA is also way off...
The alternate route screen shows that the three route options are all free of traffic, yet the ETAs for all three seem to have been calculated using a lower average speed than used to be the case - I've done this journey countless times since joining Waze, and it always used to be pretty close to reality no matter what the traffic conditions were, yet these past few weeks it's been consistently over-estimating the ETA by about half an hour. These grabs were taken on my Android client, but my wife's iPhone client was also showing the same values, so it's not client-specific.