This is going to be difficult to track down as it's intermittent and I can't replicate it, but it's happened enough times for it to be worth reporting and looking into.
When I'm signing into a website in Chrome, and I use the keyboard shortcut to auto fill the password, it opens the password box. When I then press enter on the highlighted matching account to get it to autofill that site, instead of auto filling, it closes that box, and opens the 1Password mini box in the bar at the top of the OS with nothing selected. If I close that and repeat exactly the same thing, it will auto-fill the site as expected.
1Password Version: 7.3.2
Extension Version: 184.108.40.206
OS Version: 10.14.6
Sync Type: Not Provided