Yes, you were spot on, thank you! The problem was that iCloud had somehow been switched on for Contacts? I have never used iCloud so would this have been done automatically during an update?
I looked into iCloud when I switched to Mac but it never worked? A year or so later Apple told me the Macbook needed to be plugged into mains power but it still didn’t work. Another year later they told me it also needed to be connected to wifi but I only had cable internet so I decided it was not suitable for me and went back to External drive storage. I looked into iCloud again recently and when asking if I could choose what went to the cloud I was told it would save whatever was on your desktop? Apple had also told me that my system was probably running slower because I had a lot on my desktop so the conflicting information also steered me away from the cloud and back to external drive storage. I’m sure it CAN be useful but after all the mis-information and contradictions over the years it just seems like too much hassle to bother with which is why I was so baffled when Contacts (and only Contacts?) was switched to iCloud?