Well done, Sandy, kudos to you. It was an artefact caused by the different anti-ghosting options when doing a multi-exposure blend. You can see a dark/light edge in the sky on the right side. I'm still very much learning how to get the best out of this tool. So, it's a case of being offered 4 options: off, light, medium and strong... and if one produces an artefact, just try a different setting, until you get an artefact-free result!
That's probably not how you are supposed to do it... I really should look up YouTube tutorials on it.