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):
- 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
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.