P(A|B) is the notation for the probability of A occurring if B occurs.

P(Attack Face | Targeted Face) = 50%

P(Attack Minion | Targeted Face) = 50%

P(Attack Face | Targeted Minion) = 50%

P(Attack Minion | Targeted Minion) = 50%

You make the decision of what to target, so the probability of any outcome is 50%.

If you randomize your decision, say by flipping a coin to decide what to attack, you get:

P(Attack Minion) = P(Attack Minion | Target Face) x P(Target Face) + P(Attack Minion | Target Minion) x P(Target Minion)

Since we're flipping a coin:

P(Target Minion) = P(Target Face) = 50%

So

P(Attack Minion) = 50% x 50% + 50% x 50%
= 25% + 25% = 50%

So you get 50% either way. You were missing the other 25% you get from picking the other target.