Okay, after a bit of misunderstanding things, I've figured it out:
the problem is that I'm saving the 'save as'/new file elsewhere, i.e. not in the same folder.
SC then cannot show it, so when I move to another file, the changes made are applied to the original.
This was complicated (for me) by the following:
I actually initially thought I was saving in the same folder, but I wasnt.
The default 'Screenshots' folder is
I changed this to
when I go to 'save as', it goes to the default one - I dont know is this SC's responsibility or mine ...