Thanks @jfheath. And, yes, the XT did a Crazy Ivan almost before I reached the entry point for the route.
I don't have a picture of it, but I was curious about the triangle shape.
This makes sense.Under certain circumstances - like if the closest point is behind you at Point A, it will calculate a route from where you are now (call it Point B), which involves turning round and going back to point A.
You ignore it. And carry on. The closest point now to the correct route is now at Point B. The XT seems to have modified the original route so it is now B to A to the end. You have just ignored it, so it issues another instruction taking you from C to B to A to the end.
And so on - and it may continue doing this until the actual closest point on the original route is ahead of you - and you are closer to that than you are to the last time that it asked you to turn round. You can have the situation where it is trying to take you on a 25 mile trip to get you to reach a point half a mile ahead.
When I stopped in Shere's car park I had the situation depicted in picture #3 or #4.
The car park is probably at most 50 meters from the actual route.
When I got out of it the XT was pointing me in the wrong direction (left). I went right as I know the road and after a while it started showing the route correctly. I probably passed a shaping point.
A couple of extra notes.
I have one additional map on the Zumo.
Is an OSM-based non routable map I use for going off-road. I uniquely use tracks offroad.
At the time of all the above this map was disabled. All the standard XT ones (CT, DEM, Parkopedia) were enabled.
I can tell when the OSM map is enabled by error while on the road as the speed limits disappear and turn indications are missing.
The random waypoints.
Those extraneous waypoints were there as I was testing if there is a way to have a notification if you approach a waypoint that is not part of a route as I do on my GPSmaps. It didn't work.
As I said, since I do not use audio with the sat nav, I'm not much bothered by this.
I'm traveling to Germany in a few weeks.
I'm preparing a lot of routes (and tracks). I'll start removing any avoidance for the time being.