Heart Rate Missing in Activity App
If you're seeing this message when you check out your Workout in Apple's Fitness app, you aren't alone. That graph showing or not showing is not up to Slopes. The heart rate data is there in HealthKit (you can see it in the Health app), but Apple is choosing not to display it. They have their own criteria for when to show this graph (number of samples on average per minute or something like that) and sometimes ski activities, especially with longer breaks when you pause, cause them to reject showing it.
Sadly there is nothing to do to fix this at the moment, beyond complain to Apple (which we have). In the mean time, though, you can see your heart rate data in Slopes on the Timeline Screen when you hit Vitals.