Yes, it is normal to recover what you bet with the challenge itself, and get the plus of the prize, even when sometimes the prize is small because of the people playing.
This is what I like the most of Primedice challenges: basically you participate KNOWING that you are going to earn more than what you bet.
I've personally had experience where my losses have been much much more than I could ever recover if I ever win the challenge.
The house is still in overall +ve when it distributes the prizes to winners.
Im sure that if people keep betting with that low multiplier everyone will be EV+ that means the house is EV- for this event. Overall they are still making profit of course because we are talking about primedice and they are making bitcoin from those losing gambler even now as we speak of it but in this event it is different.