Your opponent changed tactic and player.
The engine does not change tactic and he replace the suspended and injured players following the order of the subs, except If he player i S1 is a GK, then he change using the player in S2 position.

Last season i had a Team tha the manager didnt saw any game, and only had 2* players, but he chnged everyday His tactic acording to His opponent, i think this is what happen today with you.