It's well known that first round in siege is broken, due to doors breaking faster it's almost always a win for attacking team.
I've raged sometimes when after a difficult siege round you win a well earned x5 to see in next map you are on defending team and loose your x5 due to broken game mechanics.
I myself don't play first round when i have a good multiplicator, beside i wondered how greys were always in attacking team, still it's not really an exploit, more a correction of broken game mechanics.