Over the past week or so there's been some chatter around issues with the left Joy-Con occasionally disconnecting from the Nintendo Switch. While it appears to be a relatively infrequent occurrence (in many cases happening a few times over many hours, or not at all with this writer) it's the sort of issue that could become more pronounced once a couple of million systems are out in the wild.
In any case, some were claiming today that the issue was resolved by the day one update. Digital Foundry (through Eurogamer) has put this to the test and found that the system update has not addressed the issue. The article also goes into detailed tests to clarify the specifics of the problem; it's ultimately a blend of distance and whether the Joy-Con is substantially concealed by either the hand or placed behind the back. A table below provides a summary.
The full article is worth a read for a detailed assessment of what prompts connection drops.
It should be noted that plenty are declaring the issue fixed for them on this Reddit thread, so what we do suggest is having the Joy-Con attached when updating the system - we did, but that was just coincidence as we were working at a desk.
As we've said before, we're not convinced it's a huge problem due to the circumstances that trigger it, but with the system about to hit shelves it'll hopefully have a minimal impact. If the day one update doesn't fix it (though, again, some believe it does) then hopefully Nintendo will act quickly with a firmware patch.