Since using MapSource and later with BaseCamp I have created routes by placing waypoints before the intersection and a little after it if the planned route changed direction to a different road number and there has never been a problem.
I checked the mentioned roundabout in BC and on XT using only three points. While BC draws it as I posted in the previous post above, XT does exactly what @b1324 mentioned. It goes around the roundabout and returns to the point before the roundabout and turns back into the roundabout. When I remove the point before the roundabout on XT it goes correctly, but uses that side street before the roundabout.
On OSM maps this side street is not connected and looks like a driveway to two buildings.
Maybe this is where the error lies.