Fan headers have several options in BIOS Setup, but two of them get confusing.
The PROFILE setting is how the fan decides what speed to set for what temperature is reported by its sensor. It is the "strategy". Options normally include Standard or Normal (the default "fan curve" set by the maker), Turbo (fixed full speed), Quiet (fixed slow speed), and Manual or Custom (allows YOU to set your own "fan curve"). Many consider the Standard to be the default "Automatic" control setting.
Separately there now is a setting for MODE, which is the METHOD (or type of electrical signals) sent to the fan to achieve whatever the Strategy has decided. Options here normally include PWM (for 4-pin fans) or Voltage or DC for 3-pin fans. But also many include here "Auto", which MAY appear to mean "normal automatic temperature control strategy". But that is NOT what this "Auto" means. This option has the header on every boot-up TEST the fan to determine whether it should use 4-pin PMW Mode or 3-pin Voltage Control Mode. Basically it assumes PWM Mode and sends out a signal for the fan to start at full speed, then notes what that speed is. It then sends out a signal for some much lower speed. IF the fan speed does actually drop, then this must be a 4-pin fan and the MODE is already correct. IF the fan speed does NOT drop off, then this must be a 3-pin fan, and the header changes its own MODE setting to Voltage Control Mode and uses reduced voltage to force the fan to slow down to what the Profile (strategy) has decided.
So the "Auto" setting for MODE is handy of you don't want to figure out the setting for yourself. But some people misunderstand and think that is the correct way to set the default PROFILE "fan curve".
The "Auto" MODE option also causes a different problem that is NOT one that OP would have in this case. In some systems with AIO cooling one needs to connect both the PUMP and the RAD FANS to the CPU_FAN header. When that is done with a Splitter it is important to have the PUMP connected so that its speed is the one sent to the CPU_FAN header, so that the header can monitor that speed signal for possible FAILURE. BUT the pump is wired just like an older 3-pin fan, so IF this connection system is being used and the CPU_FAN setting for MODE is set to "Auto", the header will discover that its "fan" is 3-pin and use Voltage Control Mode to slow the PUMP down. Almost all AIO system want that pump to run at full speed all the time, so this can severely reduce the performance of the system. The solution in this set of circumstances is to set the CPU_FAN header to PWM Mode. When that is done, the PUMP will always run full speed, but the RAD FANS always are the newer 4-pin variety and their speeds WILL be controled properly by that signal system.
OP, I have no doubt that your AIO system has 4-pin PWM style fans on the rad, so this issue will NOT afffect you when they are connected to the CPU_FAN header. Your PUMP, if you connect to the AIO_PUMP header as you plan, will be fed the constant full 12 VDC power supply it needs to run properly, too, because that header normally does NOT allow its output signal to call for slow speed for a PUMP.