Apple Watch workout app closes when fitness app opens

I started a HIT workout on my Apple Watch in the workout app. I then went to my Freeletics app to start a workout. In the middle of my workout I went back to the Apple Watch workout app and it had closed my HIT workout. I kept trying to start a new HIT workout but every time I switched to my Freeletics app it would close the HIT workout. It didn’t used to do this. Any help is appreciated.