You may not have "solved" this completely by using the CPU_PUMP header. The why? part of that needs an explanation.
3-pin and 4-pin fans require different types of electrical signals to control their fan speeds, and if you mix them up and use the "wrong" signal that has certain effects. One of these is that, if you plug a 3-pin fan into a mobo header using the new PWM Mode of control signals, that fan will always run full speed. In most AIO systems the FANS supplied for the rad are of the PWM type and their speeds are controlled properly by such a signal system. But the PUMP is designed just like a 3-pin fans, so if it is connected to that same header and signal system, it will always run full speed. As it happens, in most AIO systems that is exactly what we WANT that pump to do! So the CPU_FAN header used to connect the system NEEDS to operate in the new PWM Mode.
HOWEVER, many mobos now allow you to set that MODE option AND often have a default setting for the CPU_FAN header to automatically set itself according to what fan is plugged into it. It does that based on a test it runs at every start-up. It uses the newer PWM Mode to send out signals to start the "fan" it believes is there at full speed, then slow it down. It watches the fan speed signal coming back. If the fan slows down, things are OK. If the fan fails to slow down, then it must be an older 3-pin fan, so the header changes itself to use that older Voltage Control Mode so that it CAN force the fan to change its speed. With an AIO system, the normal connections feed the PUMP speed back to the header. So when this test is done, it always causes that header to change to Voltage Control Mode and reduce PUMP speed because the CPU temperature is low, and this also causes the FANS on the rad to run slow. For an AIO, this is what we do NOT want! We want that pump at full speed always! So the solution is you go into BIOS Setup for the CPU_FAN header and SET that option to PWM Mode, not to Auto or Voltage or DC. Then the header WILL use PWM Mode, the PUMP will always run full speed, and the rad FANS as PWM fans WILL have their speed controlled to control cooling of the CPU.
The CPU_PUMP header is set up differently. It knows that the PUMP should be running full speed all the time for most AIO systems, so it is set NOT to try to slow down whatever is plugged in there. Thus you do get constant full speed on the pump as needed, BUT you also get constant FULL speed of the rad FANS connected to the same header! There is NO automatic control of CPU cooling according to its temperature.
Now, a few mobos have an option on some headers to select whether they behave as PUMP headers that always run their device at full speed, or as FAN headers that change their device's speeds. OP, unless your CPU_PUMP header has such an option I suggest you connect your AIO system back to the CPU_FAN header and ensure it is set to use only PWM Mode.