Health might have to do with it here. The main team I use is Lightning-GNW Kiba-Sasuke-Ukon Sakon, and whenever the opponent casts a mystery it's always on whoever has the least health. (usually either Sasuke or Sakon)

An uncultivated tank character, even with naturally greater health, probably has less health than a fully cultivated character. That might have something to do with it, but since I don't know your team's healths I might be wrong here.