I use the Pan feature in the Mixer to create 100% stereo separation between staves. This is done to create tracks for choral singers to learn with. Vocal lines are represented with instruments - usually, woodwinds and brass.
I find that, occasionally, when I export the file to wav, a single note on one staff, set to pan=100% right, leaks over to another staff, set to pan=100% left. This isn't a reverb problem - I set reverb in the performance window to OFF. I've tried turning off Sibelius, and rebooting the computer; sometimes this gambit is effective, but sometimes not.