New console hardware is often subject to teething troubles, and it would seem that the Nintendo Switch is no exception.
During our time with the system we've noticed that the left Joy-Con sometimes disconnects entirely from the console for no reason, and we're not alone - other outlets with early access to the new machine are reporting the same problem:
In our case, the disconnections occurred outside of gameplay, but others are reporting that it's happening during The Legend of Zelda: Breath of the Wild. The left Joy-Con disconnecting is quite a disaster in that particular game, as the left stick is of course used to control Link's movements. We've been playing mainly with the Pro Controller when using the TV, but even when using the Joy-Con in the grip for some sessions we haven't had a disconnect in-game.
GameXplain have attempted to "test" the issue, but bear in mind that Bluetooth technology has limited range and the presenter claimed to be sitting 10 feet away from the Switch, so we're not sure a solid answer is found here:
We'd imagine this issue will be solved by a firmware update in the future; the chances of it being hardware related cannot be ruled out, the fact that it's only the left-side Joy-Con suggests that the problem may be software-based. Fingers crossed it will all be sorted out by the time the console arrives on shelves on March 3rd.