Unless you set “preheat before arrival” to off geofencing doesn’t actually turn the heating off when you’re away from home. Have you got it set to comfort? Try balanced or economy to reduce the amount of preheat.
If you are more than 5 miles away from home and are using economy or balanced mode then I would expect the temperature to drop by a few degrees before it starts to preheat. I believe Tado support can review your geolocation data and make additional configuration changes on their servers that you cannot make in the App. Might be worth a try if you’re not happy with how the function works for your home.