Most of the actual logic board is sealed. So unless you spill something corrosive that goes through that seal and damages the logic board liquid can only get to

Most of the actual logic board is sealed. So unless you spill something corrosive that goes through that seal and damages the logic board liquid can only get to and damage[0] actual connectors between components and physically moving entities. In case of MBP physically moving entities are mostly fans and keyboard. My guess would be that grape juice shorted some of those connectors. After few months it dried sufficiently, such that when you reseated them short went away. [0] And by damage I mean mostly short and/or corrode the metal.