Characters that can apply stun on their base attack like AS and RG have them instantly removed from the opponent if the stun is applied as a counterattack. It appears the check for which effects to remove has been moved to after the counterattack and since the stun's turn count is 1, it gets -1 and then cleaned up as a 0.
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+1
It's always been like this though. It does behave like removing stuns is done at the end of a "turn"
Assist on AS/Fives counters. Happens for me all the time. Wastes the stun.