Okay, you may have to sign up for a free account on Airbnb to see this in action and reproduce, but it really drove me nuts thinking Airbnb had a bug (and maybe they still do, but a different one) until I realized 1P was getting confused by whatever DOM magic Airbnb is doing. This may very well apply to other web sites.
Airbnb like many sites has an Edit Profile page. On that page is an Email Address field which I am trying to update. When that is filled out, I press a Save button at the bottom. They use DOM manipulation to "popup" a dialog asking for my password to verify the change. I use 1P autofill for the password, but behind the scenes (literally and figuratively) 1P has changed the Email Address field back to what it thinks it should be. Aaargh.
I can struggle to get you some occluded screenshots -- but I'm really hoping that description is enough.
My guess is that Airbnb isn't doing things properly to "deactivate" the DOM elements (although everything under the popup is dimmed / grayed) and that is tripping up 1P. Hopefully you can reproduce this yourself with an Airbnb account to debug.
Workaround of course is to bypass autofill and manually copy the password and paste it in.
1Password Version: 7.2.4
Extension Version: 22.214.171.124 (Firefox 65.0)
OS Version: OS X 10.14.3
Sync Type: Not Provided