Android version lives infinitely while it plays some track, even if there are other heavy apps running at the same time . But after you pause track it is not guaranteed any more. My HTC Sensation normally kills Spotify after about 10 minutes on pause (both Gingerbread and ICS).
So any conversation or phonecall may force to restart app. With bug resetting playlist to first track it doubles annoyance.
I guess this is a phone specific problem and there's not really something Spotify can do about it. On my galaxy for example, this doesn't happen. However if you still got problems, feel free to open up a topic in the help forum about this bug. Thanks! ;)