Ah! I forgot about that! OK, understood.
That's the intended behvaiour Mark. Routes added to the background map are now "remembered" until you clear them, as per this post. You can hide them temporarily by toggling the Traces switch under the route planner's Display Options.
It seems the Trace a Route is remembered between Plotaroute sessions (ie closing the PC app but not logging out). Clearing the map (using the Clear button) defintely clears the Trace.
I don't think this is expected behaviour?