How can I stop MacOS 10.14.4 Safari from automatically logging me in when I select a password?
Background

Unfortunately it looks like Apple decided to change this behavior in Safari, at least according to this article titled: Safari: macOS browser now autosubmits logins. Here’s how to disable it:


  Apple changed the behavior of Safari in macOS 10.14.4, and you may
  have noticed it and thought it was a bug. Now, if you have stored a
  password for a website, when you select a login entry to autofill,
  Safari 12.1 for macOS automatically submits the login. Previously, it
  would fill the fields and still require you to click a Login or Submit
  or other button to proceed.


Apple mentions it here in their Mojave 10.14.4 Update release notes (2nd bullet):


  Safari
  
  
  Adds Dark Mode support for websites that support custom color schemes
  Streamlines website login when filling credentials with Password AutoFill
  Allows push notification prompts only after interacting with a website
  Adds a warning when an insecure webpage is loaded
  Removes support for the expired Do Not Track standard to prevent potential use as a fingerprinting variable; Intelligent Tracking
  Prevention now protects against cross-site tracking by default
  


Workaround

So it looks like your only recourse here is to disable auto fill under the  "User names and passwords" section of the AutoFill tab of Safari's preferences.

AutFill tab under Safari Preferences



Edit under User names and passwords



NOTE: That dialog is actually just the Passwords tab, so you can just navigate to it directly.