The Joycons are a bit of a problem- this is something that I noted about the Switch’s controllers in my own hands on with the system. And now, it looks like they may be more of a pain in the ass than believed initially- because as of right now, it seems that there is a persistent flaw with the system that causes specifically the left Joycon to lose synchronization with the Switch and disconnect as a result.
This is a problem that has been noted by many reviewers and people who have had the chance to go hands on with the system ahead of its launch, notes Polygon– but as of right now, it is unclear if this is hardware problem, or just a software bug. We do know that there is a day one update for the Switch planned, and it is entirely possible that it resolves this issue.
Still, as of right now, this is a bit worrying. Best to just keep those Joycons snapped on to the Switch (or in their grip) if you are planning on playing with them at all. For others, there is still the Pro controller.