Can’t get my Raspberry Pi 4’s fan to stop spinning full blast even with PWM control

I recently set up a Raspberry Pi 4 as a lightweight media server in my living room. To keep it cool, I installed a 5V PWM-controlled fan connected to GPIO with a transistor switch. The idea was to have it run quietly at low temps and ramp up only when the CPU hits 60°C. However, no matter what PWM signal I send, the fan immediately spins at full speed and never slows down. I’ve double-checked my wiring and confirmed the fan works fine with a simple 5V power supply. I’m using a Python script with RPi.GPIO to output PWM at various duty cycles, but the fan’s behavior doesn’t change. Has anyone seen this with the Pi 4’s GPIO fan control? Could it be a fan compatibility issue or something about the Pi’s voltage/current on that pin? Any suggestions on how to get true PWM speed control working would be awesome!