You probably have the timer set without realizing it, or you're not deactivating it correctly. The timer has four settings: 1H, 4H, 8H, and Off.
To check and turn it off: Look at the timer indicators (1H, 4H, 8H). If one is lit, the timer is active. To deactivate it, press and hold the Timer button until all the timer indicator lights go out. Just tapping the button cycles through the time settings; you need a long press to turn the feature off completely.
IF THAT DIDN'T WORK: The unit might be in Auto or Eco mode, which overrides manual timer expectations. In Auto mode, it runs based on air quality. In Eco mode, it shuts off only after detecting no pollution for 30 minutes. Press the Speed button to cycle to a manual speed (1, 2, or 3) and see if the timer then works as expected.