Thank you for your time and assistance.
The problem was that I could not see the 1Password prompt when I tapped on the email address text field on Best Buy's login page. I think some update that happened before you replied fixed it. The prompt appears and it autofills correctly now.
After that, I checked the Play Store for updates, and there was an update for Messenger. After the update, I opened Messenger, then I tried to log in to an alternative account by using the "switch account" menu in Messenger.
I tap on "Add account", then the log in page appears. I tap on the "Phone number or email" text field, expecting the 1Password prompt to appear within 5 or 10 seconds at most. But no prompt appears; only my keyboard appears.
The only way to get the 1Password prompt to appear is to press on the text field until an "Autofill" prompt pops up near the text field. I tap on "Autofill", and only then does the 1Password prompt show up.
It would be good to have 1Password appear immediately, as expected.
But maybe the problem would be fixed if I try to uninstall and reinstall the Messenger app. So I tried that, and I tried signing in to my account in the Messenger app without using the "add account" feature.
It works. The 1Password autofill prompt appears and autofills the text fields correctly.
So, the problem involves the "add account" feature in Messenger. 1Password will not automatically offer to autofill the log in page if you try to add an account that way.
