Do you actually have something selected before you go to set the Stroke color? It may be that the Stroke color was already active and you deactivated it when you clicked on the button.
Thanks Dave, as I suspected it was a case of me not quite getting the required order of things right.
I think I thought that to change the colour of something I had to click on the swatch next to the Stroke buton in Shape Style and as a result was actually deactivating the stroke colour setting as per your screenshots! Now that I get the logic of how Layout wants it selected it works everytime!
You do have to do it once to let LO know you want to edit that color but once it’s activated, you don’t need to click it again. Keep in mind that if want to change the color of something else and click that color swatch button, say next to Fill or in Text style, you’ll make Stroke color swatch inactive. And of course in order for a change to actually be made you need to have an appropriate entity or tool selected.