This can actually be caused by a bad trackpad cable or liquid damage on the trackpad connector on the trackpad itself or the logic board side.
First thing I would do is open the bottom cover of the MacBook and inspect the connector on both sides for any signs of corrosion which is usually in a white powderish form and darkened (corroded) pins.
If it looks okay I'd replace the trackpad cable and go on from there.
On the 2015 A1502s they commonly go bad as it rubs against the battery and bottom cover. Applying tape over the replacement cable alleviates this problem.