Did you add the site to your app's Associated Domains Entitlement? You need to do that in addition to adding the Apple App Site Association file to your site which you said you've done already.
Documentation here: https://developer.apple.com/documentation/security/password_autofill/about_the_password_autofill_workflow
Answer from Dima on Stack OverflowVideos
My problem as submitted to LP support:
When I select LastPass in the safari share sheet on my iPhone 11 (iOS 13 & 14), the form fill option (not username/password autofill) is not available in the LP window, only "add new password", "generate new password" and "log out" in the upper right of the LP window. iPhone Settings>Passwords>Autofill Passwords and LastPass are enabled, iCloud Keychain is not enabled. I understand the changes of where to find the form fill data with the New Experience but the option to choose it is not available. LP autofill username/password does work in Safari and other apps. I’ve verified with Safari MacOS 10.13.6 that Form Fill does work and my form fill data is there. The form I tested on the Mac is the same as the one on i tested on the iPhone, so it is not a Flash or Silverlight form. I have the same experience on my iPhone 6 running iOS 12.4.8 and iPaid Air 2 running iOS 14. I have performed Update Vault and logged out of LP on all devices, still the problem. I get the same experience using Chrome on my iPhone 11.
In iOS you can tap any text field on to reveal an auto fill option (second image) that lets you paste passwords, but it doesn’t seem to work with LastPass. Attempting to use the feature results in the first image, a popup stating that the app needs a developer update and that the feature is unavailable until then.
My autofill is set up, it works great when filling passwords on login pages, I’m just a bit irked that I can’t paste a password like this.
Why doesn’t this work? It works a-ok with keychain.
On an iPhone and have been struggling with the LP app since last update. Paid premium subscription ending in August.
Tried as I might, LP just refuses to autofill fields when in a web browser (opera, chrome, safari, Firefox). It works fine when autofilling native app logins. But when it’s a built in browser or a regular browser app, it simply refuses to autofill usernames and passwords.
I had to resort to copying and pasting my credentials for every single web based login for the last couple of weeks.
Wondering if this is a problem specific to iOS beta on iphones or is this an LP thing. Anyone else has the same problem with iOS 17 and other password managers?
This will help me decide if I should remain with LP or GTFO.
Thanks!