Let me try to explain the behaviour of a 'route' on most Garmin devices, so you can understand better:
A 'route' is a series of waypoints ( bythat, I mean either User Waypoints, or Routepoints ), which should be navigated in order, to reach a final destination.
It is assumed that the destination is 'far away', ie not a circular route.
The machine will Auto-route between the waypoints by default, so you need enough waypoints to constrain the route.
So let's consider how it works...
Let's assume a route has been created, consisting of 10 waypoints, in more-or-less a straight line.
We are navigating along it, and we are now part-way between wp4 and wp5.
The machine is therfore currently auto-routing to wp5.
The road is closed, we turn off and follow the diversion.
The machine will re-calculate the route to wp5.
It will continue to re-calculate the route to wp5 as we follow the diversion.
The diversion takes us a long and winding road, and drops us back onto our original route near wp8, missing wp5,6,7.
What should the machine do?
Should it continue insisting we return to wp5?
Or should it try to route us to our ultimate destination?
Well, how it works is this:
It will continue to try to return you to wp5, untill you actually re-join the route at a later place ( near wp8 ). The moment you 'touch' the route again, it sees this, and then re-calculates to the next wp towards your destination, wp8 in this case.
This is a sensible behaviour for most situations, but not for routes that cross, or share legs etc.
Try to consider the routes you are creating with this in mind.