add custom code to app block

[piroc_media]

In Dawn theme, I have duplicated the default "apps.liquid" block to create a custom block for an insta feed (by mintt studio)

{%- for block in section.blocks -%}
    <div class="custom-insta-block">
      <div class="custom-insta-block-left">CHECK OUR INSTA</div>
      <div class="custom-insta-block-right">
        {% render block %}
      </div>
    </div>
  {%- endfor -%}

when I try to add some basic CSS, like, 

.custom-insta-block { display: flex; }

the instafeed block fails to render.  
curiously, in the dev console, if I remove the flex display, the instafeed appears. and stays even after I add back the CSS in the dev console. 
is that expected behavior?
I suppose the instafeed app doesn't load into a flex-container.