I ran into the exact same thing. I have the same motherboard but the 240 version of the ROG Strix LC II ARGB AIO cooler. The backing plate for the LGA 1700 socket does not fasten tightly to the motherboard.
After an hour trying to figure out a solution, I decided to continue with the installation to see what I was working with and figure out a solution. After completing the cooler installation, I found the cooler was mounted very securely against the CPU with no movement at all. When you fasten the 4 screws that hold the cooler against the CPU, they draw the backing plate tightly against the motherboard and the cooler securely against the CPU. It's not the approach I prefer but it works well.