Contact Form 7 multi-column form error

I think this is mostly a CSS problem.

The container for your submit button is inside a paragraph tag which combined with your floating columns will break the way it looks.

Your website is using bootstrap so I would suggest using that instead.
Here is a guide: https://www.toptal.com/front-end/what-is-bootstrap-a-short-tutorial-on-the-what-why-and-how
(I am not affiliated with this website)

If you really want to use this method that you are currently using I would just add a CSS style to the p tag that will clear it away from the columns.
CSS to move it to the bottom of the form:

.wpcf7-form > p { clear: both; }