@Carl-Lee I'm not sure what's going on, but as I've said before, I personally take Tread and other intermediate apps out of the mix and get down to the basics that I can see and understand.... Directly upload the 1.2 .gpx file via USP to the Garmin's /.gpx folder, import it, then look at the results.
Using the 1.2 format, the Garmin (XT, XT2 or 3) should not be able to (will NOT in my testing) create a route that deviates from the route that you see in MRA as the Garmin route WILL be forced to follow the .gpx file's embeded track's "breadcrumb" to create the route and disregard the Garmin's own routing logic.
If you choose to use Tread, Drive, Basecamp, MRA or any other file transfer method other than a direct USB transfer to copy your .gpx file from your computer's hard drive to your Garmin device's /gpx folder, then you are choosing to accept the ambiguity of not knowing whether or not that the file was properly transferred to the device.
If you can't see that the .gpx file that's located on your local drive was successfully copied to the Garmin's /gpx folder YOU CANNOT know for certain that it has been transferred properly.
Before anyone slams me with "the xyz app has always worked perfectly for me" believe me..... I understand!
Even though you may not be having an issue with your particular configuration and apps (GREAT! keep doing what you're doing!), for anyone who is having issues this is where troubleshooting should begin. Eliminating variables one by one is the first step in troubleshooting a technical problem. Direct transfer of the .gpx file eliminates many of the variables. That's a fact!