Don’t go via Garmin Explore, as that is what is causing you the problem.
Go straight to from MyRoute into your XT, via the Garmin Drive app.
Here is how:
https://www.ukgser.com/forums/showthread.php/571906-Getting-MyRoute-routes-into-the-Garmin-XT-easily
I use that simple method to transfer MyRoute routes into my XT. In a word, it works.
Try it. You can’t break anything, I promise.
Report back, please.
PS The straight lines are where an application (Garmin Explore, in your case) cannot reconcile data sufficiently to align it to a map. So it just draws straight lines between the known points. In your screen shot, the Garmin Explore app was apparently doing two things:
1. Correctly offering up a track version of the route, displaying it with the blue triangles.
2. Doing its best to, I guess, display the route itself, but it was unable to. So it did the next best thing, which was to display straight lines between the known points of route.
Here is an example of a misfit, using Google’s My Route app:
As you can see, the track transferred from MyRoute into Google’s My Maps app perfectly, displaying it as the blue line. The route, for whatever reasons, lacked any data which My Maps could reconcile with the Google base map. My Maps therefore drew straight lines (drawn in black) to join up the only four points that it could reconcile, being: Start, End and two known intermediate points.
As you are learning, the transferring of GPX data is sometimes not quite the perfect science that it might be. Errors, such as you are seeing can be confusing, even if their cause is often quite simple. That being said, it is an awful lot better than it used to be.
Whichever method you use, ALWAYS check the route(s) you have installed in your XT BEFORE you leave home, by comparing them with the original route, sitting on MyRoute. Why? It is best practice. Not least, realising that there is an error, whilst you are sat at home, is a lot easier to mend (or get help with) now, than when you are sitting on the side of the road. Trust me; I have learned the hard way over the years….. and I am still learning now!