This is likely when there is no password to show on a given website, but is very confusing to the user and misleads me into thinking it's a glitch when it's just there's no login available. When I click "Logins" on the dropdown of a website, it closes/reopens the dropdown instantly instead of, say, saying there's no logins available.
I'm using Firefox 68.0.2 browser on macOS.
See attached animated .gif for example. It's frame rate is too low to catch the "flashing" part:
1Password Version: Not Provided
Extension Version: 1.16.1
OS Version: macOS 10.14.5
Sync Type: Not Provided