From what I've seem sometimes the reckoning secret triggers before the frenzy and sometimes after, i don't think that's how is supposed to work. From what i remember playing against priests the reckoning would trigger after the frenzy, maybe it has some weird interaction with apotheosys and the lifesteal changes something, can we have a fix on that?
If it has 3 attack, the secret will activate before Frenzy. Else, it'll activate if the Frenzy did >= 3 attack. The secret says it clearly, "After an enemy minion deals 3 or more damage, destroy it". He attacked. He did 3 attack. The secret triggers and dies.
I have never seen a different behavior other than that one.