If a mono lead is plugged into the left input only it will not input mono on both channels, as is the case with larger Zed mixers (ZED-12 and bigger), these also have the legend "L/M", but the Zed10 and Zed10FX do not have this functionality.
One solution is to use a Y adapter to give you two mono signals, plug one in the Left input and the other in the Right.