Hello,
I'm unsure if I should make a bug report, which I don't really know how, or posting in the forum is the first thing to do. I found a minor bug with softproofing the same project on multiples windows. I tried to make a similar thing to a BoroCG video (https://youtu.be/5-q5LOhXtvY) where he has a real time, responsive navigation window of his work. Here is how I made it:
Open your project
-Window -> New Window
On the new window go to:
-Window -> New View
Select your open project
-Image -> Properties... -> Softproofing -> Grayscale/Alpha

Enabling Softproofing on the second Window will affect the first window on a weird way. The second window will apply softproofing normally, the first window will retain a Non-softproofing state (which is the desired effect). However, any subsequent stroke on the first window which has softproofing disabled will be on grayscale. You can then enable softproofing on the first window then disabled, and the strokes color will return to normal. While the second window will retain the grayscale the entire time (the desired effect).

Sorry if I'm posting on the wrong place or not being clear enough. If so I will try to explain better.

It's already been reported: https://bugs.kde.org/show_bug.cgi?id=399723