Dumb question, but are you using a monitor that supports HDR? And is set to display in HDR?
I’m not sure how KDE handles HDR but if you’re looking at the SDR and HDR side by side, one of them is either displaying incorrectly or being converted on the fly to display in the other dynamic range.
I did it from scratch following a guide from Linux Magazine from a few months ago. I had to do some modifying and it was a lot of work, but it runs pretty seamlessly now, so the effort was worth it. I’ll see if I can find it and tell you which month’s issue it was in.