@TJF in my message above I asked if you were using a denoiser. I was able to reproduce this with interactive rendering, but only when a denoiser is enabled (Render Setup > Performance > Interactive Rendering > Denoising during rendering: NVIDIA or Intel).
The solution is simply to disable the denoiser. I am not an "AI" expert, but my explanation would be that the denoiser adds some tiny artifacts even to plain areas of the image (hallucinations) and increasing saturation makes it more apparent. Unfortunately, both the NVIDIA and the Intel denoiser are developed by companies unrelated to Chaos, so there is nothing we could do to "fix" this.