K1C side fan not proportional

I have a K1C with latest firmware 1.3.3.46 (my printer download automatically last month, in december).

I have a warping problem so I lower side fan from 80% to 40% but the flow remain exactly the same.

So I try again with 3% (on the touchscreen is not so simple to choose numbers) but the same flow.

It seems that the flow is not proportional it work on at max speed or off.

I try only from touchscreen while printing.

There is a minimum PWM for that fan at which it will run, and any numbers below that, except 0, will default back to that minimum.