Some workouts in Stack not recorded in Fitness app on iPhone

I started using the Apple Fitness app on my Apple TV last week. The workouts I did appeared in the app on my iPhone, as expected. Yesterday the first workout in the Stack I created was not recorded in the Sessions part of the Fitness app on my iPhone, but the other two were, and today the last workout (in a different Stack) was not recorded on my iPhone, but the first two were.

I've closed the Fitness app on my iPhone several times as well as restarted the phone several times and the workouts were still missing. I've also signed out of the Fitness app on the Apple TV, with the same result.

Does anyone have any ideas as to how to fix this? Thank you in advance.