In the last few weeks, I noticed I didn't have any HR data at the start of my running workout. As a result, I often have to stop the workout and start over until it works.
I use an Ant+ heart rate chest strap. I set the Wirst Hear Rate status to auto and enabled Source Switching. When the workout starts, it says External HR connected.
I also noticed when I do some weight training (without the chest strap) that the heart measurements from the watch sensor are very bad, It's stuck to the same value all the time and suddenly it goes up, or if I remove the watch and I put it again it works for a few minutes.
Here you have an example of my last run with the chest strap: