They (scene blending) have the fulcrum which you can use to modify the blended result and brighten or darken the image in that module so it can be useful to use them to sort of reset or adjust the middle gray if needed… From the manual
Classic blending modes, designed for display-referred RGB (constrained to 0-100%), implicitly define a fulcrum at 50% (gray) or 100% (white) in their algorithms, depending on the blend mode. Because scene-referred is not subject to these restrictions, this fulcrum needs to be explicitly defined by the user when performing blending operations in the “RGB (scene)” color space. The additional blend fulcrum parameter will be presented to the user when using one of these blend modes in this color space. The effect depends on the operator used. For example, values above the fulcrum might be brightened and values below darkened, or vice versa.
As for the change might be because I was using levels on the individual channels by itself and with sigmoid…this may have exaggerated the effect… The perceptual space nicely spreads color but in flat images that can be like using maxrgb in filmic and you lose contrast so you need to add it back more… I think most of the time UCS looks nice but you can also use the alternate as a contrast booster …