I understand that autofill is an experimental feature, but I’d expect that it would work for some of the most visited sites. Surprisingly it doesn’t seem to work for Twitter and instead I have to manually fill my username and password. My suggestion is to fix this and implement autofill for Twitter.
Both Twitter.com and the Twitter mobile app work perfectly for me. I would check your process.
When you go to Twitter in a web browser, does it show this ?
If it does not you should correct the URI.