So I just tested it again in a custom, this time checking to see if I got the full 3 seconds in turtle stance against minions and champs, as well as towers.
Against minions / monsters and champs it is possible to get the full 3 seconds worth of autoattacks while in turtle stance.
Against towers you lose the buff after 3 autoattacks, regardless of stance.
This makes me think that it's a problem with how damage to the tower is calculated with SotD.
Essentially the Sword SAYS you crit, but the tower is all "Can't let you do that", and the so the Sword crits anyway and the tower just takes regular damage.
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