How to set or get the triggering element upon submit a form using JQuery?
Submit buttons (and their names) are special for Drupal. I'm not sure how this changed in D8, but in D7, the submit button name was used (IIRC) to determine which one was clicked.

I would suggest adding the submit button, hiding it with CSS and triggering a .click() to the button instead of doing .submit() directly. You can then either get the checkbox value if that's enough, or use a hidden field and some JS to determine which checkbox triggered the submit event (last clicked checkbox).