The Nintendo Switch Joy-Cons appear to be having problems syncing with the console sometimes.
These connection issues were identified while playing Zelda: Breath of the Wild, most notably with the left Joy-Con. However, it would seem that these desync issues are not just isolated to the left Joy-Con, and can happen even when browsing the system menus.
GameXplain demonstrates what happens when the issue crops up (with both Joy-Cons), and explores the possible causes for it. As seen in the video, the issue appears to be some signal disturbance that happens when you move your other hand too close to the Joy-Con.
It’s not clear if those playing Zelda: Breath of the Wild have noticed the issue in portable mode or using the grip. That said, all the reports come from pre-release hardware, and Nintendo’s day one patch may very well take care of those problems.