Having a bit of trouble understanding the question.
What is Dialpad? Please see page 14 of the Owner's Manual and let us know if you are referring to the Data Dial (29).
The Data Dial is used to enter and change values - clockwise increase, counterclockwise decrease... it can also turn a *selected* parameter ON and OFF.
What does your screen show when you "use the Dialpad"?
If yes, then your Instrument needs to be serviced. Contact the store from which you purchased it or find an authorized service center near you. You can usually find a service center listing on the corporate Yamaha.com site for your area of the world using the Service Locator.
I don't have a MODX to test - but I tried to find any way to get the DATA DIAL to change the A/D Input ON/OFF state (using a Montage). I was trying to force it and couldn't find any menu item that would purposefully toggle or change A/D Input. I could only change the state by pressing the button. MODX stands to be different as the GUI has variations (vs Montage). But perhaps MODX users would be similarly hard-"pressed" to find a way to make any connection between the DATA DIAL and the A/D Input ON/OFF state.
I would guess there's excess capacitance on the I2C lines which impacts the data decode or possibly the wrong pullup resistors on I2C - either one can cause the same sort of issue. It's not the only possibility - but the two switches / controls have a common entry point to the CPU through I2C if this keyboard is similar to past implementations. Could be a simple "reseat the cables and go" kind of fix. More likely you'll end up with board(s) replaced since it's faster to debug swapping boards than looking at signals.