Straight lines

[UKenGB]

Is there any way to avoid this?

yes

• pay Google
  
• use a free of charge Online-RoutingService
  
• use the Offline Edition
  

So if a route has already been calculated, don't query again when e.g. just zooming to a different level.

That is already happening.

I tried the off-line edition and it was unusable due to massive gaps in the map coverage of Europe - I mean more gaps than maps. I gave up.

That cannot be the whole story regarding zooming. I keep having the same straight line problem when I simply zoom once from a fully routed plan (i.e route between all waypoints has been calculated) but which results in straight lines replacing all or part of the route. If as you say there is no Google query when just zooming, surely the full route would never be lost just from zooming.