I'd guess the float valve for the relevant cylinder isn't closing properly.

You can check if this is the cause by letting the bike run out of fuel before you leave it: Switch the fuel tap off and allow the motor to stall. This ensure's there's nothing in the float bowls to leak into the cylinders.

When you go to restart, open the fuel tap and allow a couple of minutes for the bowls to refill. If the motor sparks up and both cylinders run then you have identified your problem!
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