Thanks for all your help and investigations. Much appreciated.
I did some additional testing too, and it turns out that if I copy a password from SafeInCloud it doesn't get stored in the clipboard history (so that option in Alfred seems to work as expected). But as soon as I paste it in Firefox it does. This behaviour disappears though if I disable the SafeInCloud Firefox add-on/extension.
So even if I don't use the Firefox extension but simply copy and paste, it still has an impact on the clipboard history. Perhaps that's in line with your findings. I'm not too familia