Nintendo
Nintendo's help website initially (and with a completely straight face) suggested that the issues could have stemmed from something as simple as using the Switch near an aquarium or around laptops, microwaves or cellphones that are turned on.
However, it seems that the the company is now quietly acknowledging an error in the design of the left Joy-Con's internals. A senior editor at CNET has reported that Nintendo customer support fixed his faulty Joy-Con in less than one week, and appears to have changed its manufacturing process to fix the problem moving forward.
Whether Nintendo will make an announcement conceding that there was a design flaw and offer a replacement program for early adopters remains to be seen. In the meantime, you can either reach out to Nintendo customer support or stay away from aquariums.