Combination of pure flexbox and bulma-css doesn't work as expected

Hello all,
I want to build a page with a top navigation and a site navigation. The rest of the page should be filled by "content".
So I tried this approach
https://codepen.io/DrSlowDecay/pen/YzjrQRO
But as you see the content part that is designed with bulma doesn't behave like I expected (I expected all three columns take the hole "green" field). When I replace the bulma part with a pure html table it works as expected. I think bulma makes heavy use of flexbox. Am I missing some extra "instructions" for the content-div?

Perhaps somebody could help me

Greeting from Germany

Dr. Slow Decay