DIAGNOSIS The timer function on the control board is malfunctioning. This is an electronic timing circuit failure, not a mechanical one.
FIX The fix is to replace the main control board, as the timer circuitry is integrated into it. 1) Unplug the fan. 2) Remove the control panel cover (a few screws and clips). 3) Take a clear photo of the wire connections on the circuit board. 4) Disconnect the wires, remove the old board, and install the new one, carefully matching every wire to its correct terminal. You can find the board as a replacement part online.
IF THAT DIDN'T WORK Before ordering a part, do a hard reset. Unplug the fan from the wall for a full 5 minutes, then plug it back in. Sometimes the digital control can get stuck in a state, and this can clear it.
HEADS UP When you have the control panel open, don’t touch the metal contacts on the board with your fingers—skin oils can cause corrosion over time. Handle it by the edges.