Unreliable, manually started activities just stop after some time, completely random


I'm wondering what could be the cause of that. I just bought a vivosmart 4 and it's running the latest firmware.

Since there is nothing remotely similar to Squash, I just selected "Cardio".
The only customization I made was defining a HR alarm.

I manually select cardio on the watch and start it. The time counter starts, the HR is displayed and everything seems to be ok.

But after the game (usually 1 hour at least), when I look at the watch, it just displays the "home screen" and I have to find out that the cardio session only lasted 2 minutes, sometimes 10 minutes or 30 minutes.

Is there anything I could be doing wrong that would cause this random behaviour?

Thank you!