Why?

I think (I could be wrong) but the cut off is if the machine is off for 1 hour or more. When turned back on after an hour or more, a new session will begin and will not be added to the earlier session. That's the way it was on older machines and I am thinking that has not changed, given your experience.

A bathroom break of a few minutes will not cause this to happen. The same session will continue if the machine was only off for a few minutes.