Are you refering to the old XT or the newer XT 2 or even XT3.

Using the Garmin Drive app does not alter the route created and saved with Here/Garmin mode chosen. Do not use or do not stay in the TomTom mode when leaving/saving the route in MRA Routeplanner

If you prepared the Route in TomTom mode you need to compare that Route with the Here/Garmin mode. Add additional points in order that the TomTom matches the Here/Garmin road. Do not forget to put up to 29 ViaPoints. Those are the ones you can skip in the Garmin XTs. When skipping a point or leaving the route the XTs recslcultate the route up to the next ViaPoints. If you do not use ViaPoints (the ones with the hand in MRA) the whole route until the end will be recalculated.

When you own an XT2 or XT3 and you import the mra Route via the Tread App a recalculation of the route inbetween the ViaPoints takes place. So use enough ViaPoints and at them at critical places where you absolutely want to drive through. Importing the route on the SD cart on the computer helps the route not being recalculated in between the ViaPoints . Best is then even not syncing with the Tread app.