When you copy a password from the standalone app (at least in Windows):
1. after some time (which is configurable in the settings) the copied data is cleared from the clipboard
2. the copied data is prevented from appearing the Windows Clipboard manager (available by pressing Win+V)
However, when copying a password from the browser add-on, neither of these things are true. The password does appear in the clipboard manager, and is also never cleared from the clipboard itself.
Personally, I've avoided syncing my Windows accounts because I've never quite understood the security implications and it just seemed like a bad idea. However, if you do sync settings in Windows 10, I believe the clipboard manager is among the things which are synced. Having passwords on it is a potential security problem.
I think it would be a good idea to implement the same clipboard security measures already present in the standalone app to the browser add-ons.
1Password Version: 7.6.800
Extension Version: 1.25.5
OS Version: Windows 10
Sync Type: 1Password