Nintendo says "manufacturing variation" caused interference.
Since before its release, reports have been coming in about connection issues with the Nintendo's Switch's left Joy-Con controller. It was suggested that the issue could be caused by the controller's design, as the right Joy-Con features a stand-alone antenna component, and the left Joy-Con doesn't. The Bluetooth antenna in the left Joy-Con controller is not a separate piece, but rather is printed on the controller's circuit board, and it's located next to the housing for the joystick, which is a metal box. It's beneath the shoulder button, so it's not uncommon for the area to be covered by a person's finger. YouTuber Spawn Wave was able to fix the issue by soldering a copper wire to the antenna circuit in order to move the antenna to the bottom of the controller.
In a statement today (via Kotaku), Nintendo of America stated that the issues were not actually caused by a design issue, but rather were due to a "manufacturing variation" that "has been addressed and corrected at the factory level."
You can read the full statement below:
There is no design issue with the Joy-Con controllers, and no widespread proactive repair or replacement effort is underway. A manufacturing variation has resulted in wireless interference with a small number of the left Joy-Con. Moving forward this will not be an issue, as the manufacturing variation has been addressed and corrected at the factory level.
We have determined a simple fix can be made to any affected Joy-Con to improve connectivity.
There are other reasons consumers may be experiencing wireless interference. We are asking consumers to contact our customer support team so we can help them determine if a repair is necessary. If it is, consumers can send their controller directly to Nintendo for the adjustment, free of charge, with an anticipated quick return of less than a week. Repair timing may vary by region. For help with any hardware or software questions, please visit http://support.nintendo.com.