Yea, I found out the same thing too while I was doing the practice tests with PR online. The problem is that everytime you hit mark, it seems to reset your time to the amount of time you had when you first started that question. I noticed this because also if I answered a choice and then hit mark, my answer choice would go away too. We should definitely e-mail PR and tell them about this so they fix it.
For now, what I recommend doing is just not hitting the mark ever. It kind of sucks because you don't know which ones you may want to come back to, but you could always just right the number of the question you are not sure about down on a paper and in the end if you have time go back to it like that.
