Asrock Z370 Pro4, Chassis Fan 1 constant max speed

Hi everyone,

I'd like to ask for some help with an issue I'm facing with a Z370 Pro4 motherboard.
For some reason one of my case fans, the one connected to CHA_FAN1 is constantly running at max speed, at around 1150 RPM, even when my PC is idle (both the CPU and the MB temp are around 31 Celsius). The other fan, connected to CHA_FAN2 is running at much lower speed, around 350 RPM.
I tried changing the mode of the fan from Standard to Silent, but it's still running at max speed.
I also installed A-Tuning, and tried doing a FAN Test, but it seems that that fan runs at the same RPM regardless of the "Fan Power", this was the result:



I did the same test for the other fan, and that one seems to work correctly, the RPM increases with the Fan Power.



I also tried changing the source temp in the BIOS from M/B to CPU, but that didn't help either.

If it matters, the two fans are Arctic P12 Silent and Arctic F12 Silent (they both have the 3-pin connector). The P12 (used for intake at the front) is the one which is running at max speed.

Does anyone know what might be the problem, or how I can troubleshoot further? (It's the front fan which is constantly running at max speed, and it's pretty loud, so I'd like to find a solution to this.)