Previously,(and currently on the real servers) any time a mob rolled a counter attack, there would be a shield block check. If the block was successful the counter attack would be prevented and the attackers strike would fire unimpeded. Much like the accuracy check on counter attacks.

As of the recent test server update, this is no longer the case. An Ochain PLD will have capped block rate on ~99 players/mobs. And thus should be impossible to counter attack. But tests in both ballista and against MNK mobs show many counters going through.

As no adjustments to counter mechanics themselves were announced, I can only assume this is an unintended result of implementing the new MNK 2hour, "Consummate Counter."