Zumo User Forums

From my [limited] experience it appears that:

1. perfectly valid routes from imported gpx files can change dramatically when Tread syncs with the Explore database. No edit of the route is involved. It seems that the original route's shaping points are ignored when the sync and related route recalculation occur. I have a feeling that may be related to the Explore database rather than Tread itself, but I have no proof, at least not yet. We had discussed the issue in this tread: https://www.zumouserforums.co.uk/viewtopic.php?t=3007. I am still in dialog with Garmin support on that.

2. When NOT syncing with Tread (or prior to syncing), the XT2 may make minor changes to shaping points (e.g. name changes) depending on the point's subclass. We discussed that deeper in the above thread. I have not [yet] pursued that with Garmin support.

I will, of course, share whatever else I learn from further tests and from Garmin Support.

I have some (initial) proof of that. I need to add that to the link that you provided to continue that conversation.
But it only does it (so far) if the app is open on the phone. Not if it isn't open.

Your point 2 - the minor changes to routes also happened on the XT - I think it was garmins way of dealing with the fact that so many people were having problems using third party mapping software and route points were not plotted correctly. I think what is happening is that the coordinates are taken as given, and the decimal number for the coordinates is reduced from say 8 decimal places to 5 or 6, rounded off so that they are not as accurate, and then the nearest known place on the road in the Zumo database is found. In towns the resulting change is very small, but on the open road the nearest known point could be a good way up the same road.