Groove mosfet doesn't disconnect

I bought a groove mosfet v1.1 to turn a small peristaltic pump on and off.
Website for the groove mosfet v1.0
The pump needs 12V, which is my input for the mosfet. I control the mosfet with 5V from my Arduino.
When setting the Pin to HIGH, the pump runs as expected. With the pin set to LOW, the pump turns a bit slower, but still turns, so the mosfet doesn’t disconnect correctly.
If I am getting this right, there is a 10kOhm pull-up resistor to pull the voltage of the gate to VIN of the Source when switched off. As far as I know, when Gate = Source voltage, the mosfet should disconnect.
But why is there still voltage on the output side when setting the pin to LOW?

Thanks in advance