Ah, now I know what you mean. I think it's a bug or a feature.
You can start a new solve with U U'
I think that's a similiar combination to DNF a solve. In the beginning I had this all the time but recently I didn't and I also couldn't reproduce it using such combinations. Maybe they fixed it.
I think in your case it's the timeout of 5 minutes. I didn't see that before. After 5 Minutes it stops.
Maybe you should also try
https://app.cubeast.com
I think they don't have a time limit. But I also think after a couple of solves you will easily stay under 5 minutes