.list-group-flush doesn't allow nested "normal" .list-groups with borders

[Dassderdie]

• Operating system and version: Windows

• Browser and version: Chrome 81.0.4044.122

• Description:
  I want a .list-group (with border) in an item of a .list-group.list-group-flushed:
  

Instead it looks like this:

• Reduced test case: https://codepen.io/Dassderdie/pen/yLYXJNv