Once you’ve updated your Mac to Mojave and have the MacOS 10.14.4 latest version of Mojave, you’ll notice that Safari now provides the username and password for your websites as it did in the past (if you use the feature) but now it will automatically press Enter/Return.
If you’ve recently changed your password, and it hasn’t been updated and saved to the browser yet, you’ll select the login username (perhaps a long email address that you don’t want to manually type) but then instead of being given an opportunity to type in your new password, the browser will immediately attempt to log you in, resulting in a failed login attempt.
Captcha systems will fail because you’ve not been given a chance to validate those.
Assuming the browser provides the correct password, you won’t have an opportunity to click on checkboxes indicating “please keep me logged in” because you’ll be logged in right away.
According to a discussion thread on the Apple support forum, there is no way to fix this.
What’s disappointing about this bug, as with others, is that it’s easily identifiable as a user interface problem within minutes of using the new browser. This suggests that little if any real world testing was done. This happens a lot. Apple will release a new version of something, and users will immediately identify a user interface problem. It’s as if nobody is actually using the software they are developing.