It isnt a bug for me. yes they begin to shoot at same moment with same health+shield but they didnt have the same «precision score» so his shotgun deals more damage at the first shot and give him the win.
(EDIT: the precision score of your shotgun at start of the final fight is 0 and it is 0,50 for the other SG, that make him reach the 14 bullets touching target (for the kill) before yours)
The report i see in this match is the trick for secure the ressource with your shotgun (when it is a real intented behaviour). I was sure that GFX47 forbid it (or atleast he is trying to make this trick dont work anymore)