If you insert "stops" in your Streets trip, as happens if you force the trip
onto a specific road it doesn't automatically want to go onto, the .gpx you
send to your Garmin (I am using a Zumo) is not useable as a Garmin Route. The
"stops" are not incorporated into the Route built by the Garmin Route engine.
They are listed as stand-along entities.
So far the only Streets trip I can get to load faithful to my Streets trip
are ones where I have accepted the pathing Streets picked on its own
completely (i.e., have inserted no "stops" to override the selected roads).
This is unfortunate because it prevents me from using Streets to plan rides
as a ride captain for my MC, unless I am willing to break the trip up into a
number of short distant pieces in order to go on the roads I want. Which
makes me juggle many Garmin Routes during a ride.
Microsoft and Garmin need to work this out, and provide us with free updates.