iOS 11.3
There are 2 new security features.
1. A user has to click inside the autofill fields, the arrow (interaction) for the autofill to work.
2. If the site is not a secure one, the URL text will turn red in the search box..
Quoting from the below linked HowTo article:
Safari Login AutoFill
Available for: iPhone 5s and later, iPad Air and later, and iPod touch 6th generation
Impact: A malicious website may be able to exfiltrate autofilled data in Safari without explicit user interaction.
Description: Safari autofill did not require explicit user interaction before taking place. The issue was addressed through improved autofill heuristics.
CVE-2018-4137
https://support.apple.com/en-us/HT208693
At present this may not work consistently across all sites that require login.
From my experience, a combination of tapping the AutoFill fields, tapping the arrow button, entering the
info manually and tapping the "enter" key helps.
I am posting this as a user like you.
Note: This doesn't represent Apple's view on this, just my own view.
To send a Feedback to Apple: https://www.apple.com/feedback/safari.html