I was looking around for this problem as it happened to me as well.
Some looking around and disassembling my own keyboard showed bad connection on PCB to L_Alt solder point. I found a thread on geekhack that teaches to fix it by taking off solder, fixing the connection with copper wire and re-soldering. It is not necessary to take off solder.
I scratched off green isolation from the connections and just soldered them to original solder point and it works like new. No need for copper wire or to cut off original solder.