This can happen when the touch pad, light box, are not positioned correctly
Which causes the touch pad to be pinched changing the normal capacitance of one the touch pad areas.
1/16" or less can make a big difference in functionality.
This can also happen if components on the touch pad are damaged or missing.
You can go into the I/O Tests, Input, Door, and select touchpad to test it
When you do when you get to one of the touch pad buttons and it shows a logic 1 when you are not touching that area.
That is the one that is causing you the issue they all should be 0 when you are not touching that particular area.
the order of the buttons is
1 2
3 4
The touch pad and light box are about the same size.
Try to center the denom label on the glass and then center the touch and light box on it.
hopefully the adhesive is still tacky enough to hold the touch pad and light box in place as you re-assemble everything.