
Do you have the track with the fluctuating bpm set as the master track or the track with the steady bpm? It sounds like when you use Serato offline, somehow you have the incoming track (not fluct) set as the master track and when you're using it with the mixer, you're setting the fluctuating track as the master. Try messing with which track you activate sync on in the screen on the S11 and see if that fixes it.