Since you have two systems and one is working well, do you mind swapping fan and/or board to isolate the issue?
I tried this and both systems are now running fans at full speed..
I have actually come to the conclusion that it is a "low" temperature issue. Systems are burning in in a warehouse that is between 5-10 degrees C. I believe the system is running the fans slowly as a result of the cold temp and runs them so slow the fan speed drops below the minimum fan sensor threshold.
This makes the system think it has a fan failure and it's reaction is to run the fans at full speed to compensate, including the one it saw running slow.
Maybe this isn't an issue or maybe the FRUSDR needs an update to take Northern European Winters into account by lowering fan thresholds.