I think the problem is that was where someone's guard was originally and because you are using a heavy their recover could happen almost instantly right when the shot lands causing it to fail.
Maybe we have to pay attention to where the guard was prior to the shield break.
Then again I'm not sure if their guard stance stays active after a guard break. I may just be pissing in the wind
I've been seeing this a lot too.
I think it has to do with the horrid P2P servers.
I've also notices large variance in guard break and parry timing based on the host.
I've even had matches where I knocked people off the map only to have them teleport back on the map again.
I think a lot of it has to do with my connection though. Today I move out of the hotel i've been staying in and into the new apartment. I'm hoping that this will resolve my issues.