The bounty vouchers are stored in your ship log.
When you go to hand them in, the station will pull the data from your ship for confirmed kills.
If your ship is destroyed then so is the data and anything you were carrying with it.
It works the opposite for bounties on your head. As you are the pilot, you are responsible for the actions. So if you become wanted, the signal is sent out to everyone in the system to say CMDR X is wanted. Not CMDR X's ship is wanted.
So that's why your bounty doesn't disappear when your ship is destroyed
When you go to hand them in, the station will pull the data from your ship for confirmed kills.
If your ship is destroyed then so is the data and anything you were carrying with it.
It works the opposite for bounties on your head. As you are the pilot, you are responsible for the actions. So if you become wanted, the signal is sent out to everyone in the system to say CMDR X is wanted. Not CMDR X's ship is wanted.
So that's why your bounty doesn't disappear when your ship is destroyed