We repair macbooks, laptops, drones, automation controls, game consoles…electronics. What we have learned is to always look at the simplest most likely culprit first: The moving part, the power supply, connectors, use our senses to investigate these areas. And there it is! Someone spilled sticky soda in the rail and the ribbon cable - connector board / contacts area in the switch on one side is corroded. Joycon tested fine. We had to disassemble the switch (It’s easy). pull that rail and ribbon cable, disassemble it. A roll pin needed tapped out and connector header came out of rail. Inspection under microscope showed corrosion. CLR soak and small bristle brush should handle this. Will burnish contact points that show arc blackening - using fibreglas burnishing pen and deoxid, clean with 99% isopropyl then smear some dialectric grease on them. The issue on this Switch was Joycon worked wirelessly only, connection into Switch rail never detected.