This is not a typical behavior and also no known "bug". There might be some other signal routed to your channel. Could you perform a reset of project and scene (separately) and a Master Reset of patching? This procedure was called "zero out" formerly. It resets the functions of board and should clear the problem. If not, you have two options:
1) The FX Button Reset:
"StudioLive Series III Console - Factory Reset"
2) The Home Button Reset:
"StudioLive Series III Console - Reverting to the Factory Firmware Image"
Always store important settings BEFORE doing resets. ;-)