Is this the place where you want people to post about problematic sites? Or is there a place to merely "report" such things without making a public statement and occupying your time responding directly?
With Safari, and with Firefox using the classic extension, if you aren't on the Twitter home page or the Twitter Login page, but are just browsing pages while not logged in, you can click a "Have an account? Login" button in the top right, and it pops down a little window. Very hard to auto-fill it.
You can try CMD+\, but the Twitter pop-down closes, but if you open it again the username did get filled, but I haven't managed to accomplish the same with the password field.
Right-click > 1Password has similar problems.
Meanwhile, when you try to fill the password field, 1Password is left on-screen with the Autofill button enabled, but the pop-down and password field disappear. Seems a bad time to proceed to click the Autofill button. Are there any safeguards, or is there a risk of filling some non-password field? And what are the consequences of that? I suppose there could be many, just as bad as typing a password in the wrong place.
1Password X with Firefox works better for this unusual login. It fills the fields and the Twitter pop-down remains open the whole time.
Anyway, one way you can test this out is just directly go to twitter.com/someonestwittername without first logging in.
1Password Version: 7.3.2
Extension Version: 7.3.2
OS Version: macOS 10.14.6
Sync Type: 1Password