EDIT 12/7/20: At some point, this got fixed! Both Relentless and S&B now apply their RCR bonus to the mitigation gained from Crimson.
I confirmed that CC doesn’t give you any additional mitigation from the RCR you get from Relentless or S&B.
Relentless:
In this video, you can see the damage I take before and after hitting the 35% life trigger point for Relentless. The average damage I take from the tower per hit is 106.5k, and after Relentless triggers, average is 56.7k. That’s roughly 50% mitigation, not the 75% I should be seeing from all that extra RCR (which should provide an additional 50% mitigation, i.e. 75% mitigation total from Relentless.
Sword & Board
Same deal. Extra RCR shows up on my sheet, but provides no extra mitigation. Average damage per hit without S&B active: 204k. With S&B active: 143k. That’s 30% mitigation, way less than the total 44% you should get if it were considering the added RCR.
I also checked to see if your Paragon RCR works to reduce damage. It does.
Edit: also works with Topaz.
Edit #2: on a related note, I checked to see if CC’s damage increase related to CDR was updating properly when using Gogok (thinking that perhaps CC doesn’t recognize sources that change state frequently, like Gogok, with its higher or lower stacks, Relentless, with it being contingent on having less than 35% life, or S&B, contingent on having a shield equipped).
But, as my Gogok stacks increased, both my sheet CDR and my actual damage increased. So, the damage increasing part of CC appears to work one way, and the damage reducing part works a different way…