As far as Nintendo is concerned, they are claiming that there are no widespread technical problems as far as the reported Joy-Con connectivity issues are concerned. However it seems that maybe the company could be quietly admitting that there could be an issue with the design of the antenna, particularly in the left Joy-Con.
According to a report from CNET, they sent their Joy-Con back to Nintendo for repair. When it came back and they opened it up, they noticed that there was an addition of a tiny black piece of foam on the circuit board (bottom right) of the controller. Speaking to Bill Detwiler of TechRepublic, it was revealed that this piece of black foam is more than likely a piece of conductive foam.
This foam has been specially treated with nickel or copper, or both, so that it can shield electronics from RF interference. This foam is also used in instances where there isn’t space for more traditional shielding, or in the case of the Joy-Con, it appears to be a quick, cheap, and relatively easy solution for Nintendo compared to recalling all the controllers and coming up with a new design.
In CNET’s tests, they found that the foam actually did help with the connectivity issues as removing it saw the problems return. Interestingly enough it was also discovered that newer models of the controllers did not come with the foam or have the desyncing issues, suggesting that maybe Nintendo might have already fixed it for its newer models.