OK, I think I've figured it out. When you pick a location in the Maps app and then tap the blue Directions button, you're presented with a number of route options, one of which is suggested (dark blue) and some others that are options (lighter blue). This is the stage where I've been zooming in on a particular route and trying to follow it, only to have the map zoom out after a couple of minutes. Here's what I believe is happening, and why I believe you have to take one more step:
At the first screen, with all the route options, the app is waiting for you to choose one and then tap the green GO button. If you don't tap that Go button, the app eventually zooms out again to show all the routes. I hadn't been tapping the GO button because I didn't want turn-by-turn instructions, but I see now that by tapping it we can get to the functionality we want.
Try this: pick one of the routes, then tap the green GO button. The screen will zoom in to your location and the first turn will be displayed at the top. Now here's the secret: tap that first turn instruction (that's right...tap on the black portion at the top of the screen) and turn-by-turn will be suspended. You can now zoom the map to any level of detail you want. There's one minor drawback...if you zoom to a portion of the route that doesn't include your present location the map will shift back to show your present location within a few seconds, but your zoom level will remain the same. Since I usually want my present location visible on the screen this is a limitation I can live with.
So, to sum up, in order to stop the Maps app from zooming out to show all the route options, pick a route, select the green GO button, then cancel turn-by-turn nav by tapping on the first direction (in black, at the top of the screen).
