I recently upgraded to the new 1Password (v.7.3). There are two instances when the main 1Password app suddenly appears where it didn't before.
I open 1Password with my password. I hide the application. I then leave my computer for some time, long enough to require my to enter my password again. Open a web page, hit cmd-\ to enter the password. This causes both the popup password window (expected), but the main application becomes visible. Since I don't want it, I have to again hide the password. This is new behavior - the main window should not become visible until I switch to the application.
Open 1Password, again entering the password to unlock and hide the application. Select 1Password icon from the menubar. Again, both the menubar window and the main application become visible. The main app should not appear in this case - I didn't call for it, and I have to go through the trouble to hide it again with extra keystrokes. The same applies to when typing cmd-\, even when unlocked, when there is no web page to enter a username/password.
1Password Version: v7.3
Extension Version: 7.2.6
OS Version: 10.13.6
Sync Type: iCloud