I wasn't able to replicate the issue, at least with no Bluetooth devices connected. Everything worked fine on the debug build for 1.2.5 and the current 1.3.1 release in this state.
Discussion
I'm thinking it may just be the minimum required distance of walking where if you're only in like a small few hundred foot space it won't do anything but if you're going out and actually walking a good distance say like 20-30 feet and you're continuing on it works fine. Anything below that within a certain range of the last ping of the GPS just gets determined as being a point of pause so it does not get counted.
If that's the case, then is a UX problem where we're not telling the user that the session has been paused or they're being considered paused and not active. Even though the counter is still ticking.
If anything though, it could have just very well been gap in GPS connectivity or interference. That's a possibility.