If the AoE of shock clench depends on the initial damage of the the single target skill it's probably not supposed to crit. Like the AoE damage of the tri-focus passive on a heavy attack on a lightning staff it's not supposed to double crit.

Edit: I noticed some variation in the damage but that was probably due to the new concussion effect. Doesn't look like enough to be from the crit damage on your initial damage. You are right this does look like a bug.