In one of the recent updates to 1Password, something changed that the Safari extension started to open sites in new windows instead of new tabs as it used to. I found this new behaviour fairly annoying, but had just put up with it until today.
I spent a few minutes trying to find a checkbox to tick or something in the preferences, but couldn't find anything. After a while, and by mere accident, I found how to get it back to normal. If you also want to change this, just open the extension and click on the title "Open Login in…" as shown above. It'll cycle through open in new tab, new window, and current tab.
I think it's completely unintuitive, but it does the trick.