I have the same issue on a Surface Pro 6. Brand new. looking at this thread I believe it only happens when the Surface pro is charging. when on battery and fully charged the trackpad appears to behave.
Feels like an issue when under charge. charger is the Microsoft one that came with the surface pro 6.
This is very frustrating as I am now considering if I should replace the signature type cover, the surface pro or simply return both and buy an apple device.
Shame as I do like the SP6.
Any thoughts on if its worth trying to get replacements?