No, the problem doesn’t affect the normal sky sphere and I’ve tracked the problem down to the colour grading - if I disable it (set it to 0) then the problems no longer occur.
In investigating this further I’ve noticed that throughout the entire cycle I can see the colour grading (when enabled) pop between colours but that the orange of sunrise/sunset makes this all the more noticable, so if I set the time units to make the sun move slower than default I can actually watch my shadow pop between orange and black.
Also if watching the sky, the sky will flash with each colour change so won’t smoothly transition between the colours, here’s what the sun looks like as it colour changes - I can’t show a picture of this happening at sunrise/sunset as the sun becomes so blurred as to match in with the sky colour and be difficult to really see. The shot on the left is the flash, the one on the right the normal sky.
It’s almost like the sun has inverted itself.
Also, if I rebuild the lighting this can fix it temporarily (not always though) and allow me to use colour grading, however if I save the level/project, close the editor and relaunch the editor again the problem has returned as soon as I click Play to rest.