It's to do with the snapshot changes, due to how animations work on BRD and also MCH, SAM has a very easy time to just wait for you to make the first move. Any attack with a generally long wind-up animation like WAR's Primal Rend can also be a death sentence for someone since you're locked into committing the attack on the target they can just pop the buff before your animation finishes and guarantee a kill on you.
Generally speaking you'd have to be mindful of the SAM's LB timer, if you've seen the SAM use their LB quite recently then you can confidently fight them even with their buff up. Otherwise, you have to pretty much play reserved and avoid any damage interactions with them until you have seen them use their Chiten (1shot buff) and it has fallen off. If you have backup, you could try tossing in your silence skill since it does no damage and would be applying pressure on the SAM to either get away or die - the only other factor would be if you have players that can apply protective shields to you, which would let you poke and absorb the SAM lb without dying provided you're at full HP with a shield on top, no matter how big or small it is.
tl;dr - BRD/MCH is paper, SAM is scissors and counters your existence.