Hear rate sensor not found when riding a pre-loaded track. Works when using own navigation

I have connected my Forerunner 245 watch to my Edge Explore 2. When I enter a new destination into my GPS, then everything works and I see my heart rate on my GPS (as long as I start an activity on my watch).

When I create a new route using the Garmin Connect website and I send it to my GPS then my Forerunner 245 watch is not found.

What's wrong?

Thx!