anticlimactic

/ˌæntiklaɪˈmæktɪk/
adjective
  1. Causing disappointment because it is less exciting, dramatic, or important than expected, especially after a buildup.
    • The final episode of the series was anticlimactic, with a quiet ending that left fans frustrated.
    • The movie's big reveal was anticlimactic because the audience had already guessed the twist.
    • After months of training, the race felt anticlimactic when it was canceled due to rain.