Caustics denoising

[fixeighted]

I just noticed that if I raise the min subdivs then caustics are not getting denoised although everything else is. So in the end, areas with no caustics are denoised fully, whilst the caustics remain very noisy.
Is there a reason for this. so I can understand better?
The reason I raised the subdivs is in line with the docs suggestions https://docs.chaosgroup.com/display/VMAX/Caustics

EDIT: I also noticed that although the caustics RE has a denoise option, that RE when inside the B2B does not have that option - just thought it may be something to do with that...

EDIT: Ok, so it does eventually denoise...just takes until almost the end of the render and is a bit rubbish.
So presumably the way to go is to denoise only the RE.

Side note is that the fireflies caused by the caustics just add to the horror end result, so I guess that still needs work

[^Lele^]

If you render fixed, there is no noise measurement done, and so the noise level RE is empty until adaptivity kicks in.
As to rubbish quality, i'm not quite sure without seeing an image: what went wrong?