From what I've gathered - the antennae in the Joy-Cons do not get enough power. GameXplain did a video showcasing the issue. The right Joy-Con also has the issu

From what I've gathered - the antennae in the Joy-Cons do not get enough power. GameXplain did a video showcasing the issue. The right Joy-Con also has the issue, to a lesser extent. Nintendo might be able to adjust the gain on the Joy-Con radios, at the expense of battery life. They might have to re-certify with the FCC to do so.