If the tempo of the song keeps accelerating or the key changes in every chorus, you have the Practice settings turned ON.
iOS/Android: Tap the ‘practice’ button (graduation hat) in the player controls and drag the tempo slider to the left to turn it off completely.
Mac: The practice settings are in the Player Controls on the right sidebar (or bottom menu on older versions). If the Player Controls are minimised, then View menu/Maximise player controls, or click the Minimise/Maximise icon top far right. Drag the tempo slider to the left to turn it off completely.