With A&H systems, when you mute a channel, it mutes all sends. If you are hearing that channel through the main PA when you mute it, it is likely double patched somewhere. IE the source for channel 16 is also set as the source of another input that isn’t muted. If you have double checked and can say with 100% certainty that the source from channel 16 is not also patched into some other input/channel/buss, then it might be a hardware issue. IMHO, the odds are very low that this is the case however, and I think you will find something double patched.