Quoted from DaveH:

But it isn’t the same part, it’s a cheaper one. If you pop open the door of a SAM game (depending on when built), you see 4 switches on a metal plate with soldered wires. Do the same thing in a current game and you find a circuit board with membrane buttons snapped into a plastic bracket. Reduced cost and vastly reduced assembly time.
I haven’t looked through all my games to this detail, but I assume they changed fabrication houses and ended up with a bad batch that have some contamination on the boards.
This is the normal continuum where you cheapen a part until it cannot function at the base standard, and then you go up a tiny bit and start building right above the failure point.