When using the denoising node I happen to have some distortions where the light bounces. When I increase the samples to 600 it's reduced but it still be noticeable.
This is entirely normal. With too few samples, such as lower than 500 in this case, the denoiser doesn’t have much data to work with, and the result ends up splotchy as it tries to fake information. When more of the pixels are closer to their correct values, the faking is not as obvious.