This is with regards to mobs with distributed attacks, like Dern.
When the defender doubleblocks the tank, two blows get blocked for the tank, by design.
But the distributed attack onto the defender also is getting blocked.

I do not have a log but it goes like this

Doubleblock

You block XYZ MOB attack against tank.
You block XYZ MOB attack against tank.
Drakgrak blocks XYZ MOB attack against you.

Doubleblock is for defending others, so maybe this should not be happening.

I will try to post a log when I can.