Yes, dropping networkconnection causes the map to reload incomplete, and if you subsequently enter the white zone the wazeclient will start a recalculation. If you still are without connection to the routingserver the client will calculate a route based on the fragments of the map that are in memory and without actual trafficinformation. Such routes are always suboptimal... You can avoid white spots in your map by zooming in and moving the map around. All mapdata is then stored in memory and available when you enter a zone without reception.
If you do have a networkconnection and waze starts recalculation of a route (maybe because you left the planned route or because a road is not drawn precise enough in the map) the new route will always be the fastest based on your current location. If you calculate 3 alternatives you will see that the new route is always the #1 on the list. If you selected a different alternative the 1st time, that route is forgotten and history after a recalc.
A better solution would be to (let the client itself) calculate a short deviation to bring you back to the previous selected route. You are guided around the roadblock ahead of you but you keep the general direction of your route then. Alas, we have not heard that Waze is considering such an option.
AM for some 25.000 km² around Maastricht, the Netherlands
iPhone 3Gs / iOS 6.0.1