You're absolutely right, I hadn't noticed the difference between "highlighted" and "selected". I'm sorry for saying that it was bug. Yes I did do a backup first, and I'll make sure that I always do - as you say "you never know what you'll break if you don't". Thanks for you help.
I've just installed CCleaner v2.12.660 - In Registry, I did "Scan for Issues", then highlighted some issues, then "Fix selected issues", then "Fix All Selected Issues". But it fixed all the issues, not just the selected ones. Fortunately, I had done a backup and got everything back, but this is a bug.