Add custom code to app block

Technical Q&A
1

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 -%}
    
      
CHECK OUR INSTA

      
        {% render block %}
      

    

  {%- 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.