Ah thanks for clarifying!
I noticed that besides adjusting the percentage for that sss, that increasing its size along Z also removes the issue.
It becomes invisible when I change the size to 100k x 125k, for instance.
I have no idea why this happens, as the percentage basically means 50m falloff, which is not super small in absolute size.
However, I wonder though if this is really worth figuring out and all the mental exercise, please see image attached (bottom is difference between 2 renders).
There's very little difference between having this layer masked or unmasked.
Personally I wouldn't bother with this, but I'll leave that up to you...