Connection issues experienced by the Nintendo Switch’s left Joy-Con seem to stem from the way they’ve been made, a YouTube channel recently deduced.
Spawn Wave has torn the left Joy-Con apart, revealing a different configuration than the stable right Joy-Con. The left-hand controller has its Bluetooth antenna soldered directly onto its motherboard, unlike the right-hand’s dedicated antenna. The right Joy-Con needs this special configuration because it also houses an infrared camera and amiibo NFC reader in its innards.
The video below shows how you can fix the issue… providing you’re good at soldering.
If you’re not game to try, you can have a go at troubleshooting using Nintendo’s official support tips. Don’t expect great results, however.