Sometimes world class coaching makes mistakes and this is normal. They might use a strategy that might not work in a particular match and this was what happened to Thomas Tuchel. This doesn't mean that he has lost his tactical competence, even Pep Guardiola sometimes makes fatal mistakes.
Leverkusen continued with their unbeaten run in the Bundesliga but they failed to increase the gap between them and Bayern Munich. Xabi Alonso's side would have maximized the loss by Munich by winning the game against VfB Stuttgart, but a draw is still better than losing the entire three points.
You are right because even though the coach has good tactical, there are still times when they make mistakes so their strategy is easy for the opposing coach to read. This defeat was quite unnatural because Bayern Munich was beaten by a team that couldn't win the last five matches and even though you mentioned Pep Guardiola had also experienced the same thing. Thomas Tuchel will play the match away to Old Trafford and maybe he needs to restore the morale of his players because even though the defeat did not affect any results in the Champions League for Bayern Munich.
Stuttgart provided equal resistance to Leverkusen and they also played more carefully so that not many goals were scored in the match. A draw was enough because Leverkusen even conceded first and were lucky they were able to equalize so they didn't lose two points at Stuttgart headquarters.