I've ran into this bug twice now (I don't go to Mars often). You can buy the bounties but no matter the weapon the kills just aren't being tallied. I've checked to make sure I'm using the right element, even going so far as to use Hardlight, and nothing is registering kills. It's getting to be a bounty to avoid, what with them not working correctly. I have no error message to go with this as there's no crash relating to it.