Safari password autofill also executes the login button.

This is a new feature introduced in the latest update.

"Streamlines website login when filling credentials with Password AutoFill"

Section: Safari

What's new in the updates for macOS Mojave - Apple Support

This has been driving me nuts - only workaround I've found for sites that use Captcha is to manually copy and paste my user name and impossible to remember password each time. Or switch to a different browser. I don't want to turn off password autofill because that will complicated logins to sites that don't use Captcha

Others are seeing the same thing.

If you want to, you can send Apple feedback. They won't answer, but at least will know there is a problem or a suggestion for change. If enough people send feedback, it may get the problem/suggested change solved sooner.

Safari Feedback

I agree, it's a problem with some sites. Great when it works, frustrating when it doesn't. It would be great if, when Apple fundamentally changes the way something has worked since time immemorial, they would give us a way to turn off the new behaviour.

You are welcome.

I've already left my feedback at https://www.apple.com/feedback/safari.html. I suggest others do the same.