It is because you’re doing this on a Container element and there’s a bug with Flexbox Space Between and Webflow containers: it does what you’re witnessing.
Simple method to get what you want:
drag a div inside of the container, drag everything that was in the container inside the div, make the div width 100%. Now apply your Flexbox settings to the div and you’ll get what you expect.
I forgot I was making a list of steps, there’s no more steps.