Created attachment 144193 [details] gif SUMMARY As shown in the gif, after "Convert Layer Colour Space", the first square of the color picker flickers crazily. The two squares at the bottom are normal.
You don't really give any details on what you did, but I can't reproduce anything like that. It looks like you have Pigment.O installed. Could you please test with all 3rd party plugins disabled? To me it looks like something outside the selector overwrites the color as you try to select it...question is, does it come from Krita or foreign code?
My mistake. It seems that pigment.O makes it work properly only in rgb space