I thing it's a bug, or more like a mistake. If that's the case please fix it, otherwise please add remaining cell colours, I've tried to make a workaround (see a stripe going through first row on screenshot), but it's a nightmare to set up, when I need to dynamically increase or decrease number of steps (inputs) and everything stretches to fit.
Also, it could prove useful to add 32x16 midi matrix. I understand why 16 is a limit for outputs, but voices can multiply inputs.

