Stop Safari 12.1 from auto-submitting forms
I have this issue too, and I've just spent a long time with an Apple Support chat agent.

This is a "feature" introduced in Safari 12.1.

What's new in the updates for macOS Mojave


  Streamlines website login when filling credentials with Password AutoFill.


There is currently no way to turn this off. This is the solution the agent proposed. Submit a feedback to Apple:

https://www.apple.com/feedback/


  ...submit your feedback on the feature and your experience so far.


Maybe if we kick up enough of a fuss? It is a pain, and it also screws with captchas quite impressively...