fenix 5 - round counting

Hi together !
I'm a proud owner of a fenix 5 and I'm really happy with it. But one thing is confusing me - when I starting a MTB activity, after around 25 minutes the watch says "first round finish" and starts the next round. This will happens more ore less ever 25 minutes (I think it's not time related rather by distance).
Where can I de-activate this round counting ? I don't drive rounds ... ;-)
Thank you
Alfonso