Is the Pad that is sending the message called “Self”?
A possible explanation could be that the linked parameter that you are using belongs to different control. I think this could happen if you copy&pasted the Pad.
An easy way to check is to take a look at the JSON tab and see if the parameterNumber matches. If not, you can make an adjustment directly in the JSON editor or use the visual editor to link the required parameter and remove the other one.
Yes, that is correct. There is nothing that would automatically update the message definition. I agree It would make life easier.
One thing crossed my mind now, your message will turn the Aftertouch on and off. It will also reset other bits to zero. ie. pressing the pad will affect portamento, volume, etc. If you wanted to have this done correctly, the pad must link all other parameters that are used in given sysex byte and set their bits accordingly. Kind of like you did with the “Self” parameter.