As I pointed out earlier, the joy-con 'de-sync' issue cannot be fixed by firmware. Look at the teardown picture:
The part I circled (a grey cable ending up in a metal piece at the side of the battery) is what looks like an antenna. Look at the other side, it doesn't have that. Here in this picture, the blue color one is the right joy-con, the red is the left. (It's confusing, I know, but if you look carefully, the buttons and joy sticks are below the battery and PCB board, the other side is just a plastic cover. So if you imagine the joycon closed back together, it's a blue-right red-left combo.)
So, the left joy-con does not have an antenna! My theory is that the left joy-con is only connected (wirelessly) to the right joy-con, the right joy-con then connects (wirelessly) to the Switch console. (This is like left nunchuck connects to right Wiimote, which then connects to the Wii.) They probably designed it this way cos the 2 joy-cons would more likely be closer to each other than to a docked Switch. The cause of the problem will be this wireless connection between the 2 joy-cons having weak signal. (It's possible that this inter-joy-con connection is FM, which is cheaper.) To actually fix the problem, they will have to change the connection between the 2 sides (it's not likely that a firmware will magically strengthened the signal) or add an antenna to the left joy-con to make it connect directly to the Switch. These require actual hardware modification.