Titlebar Separator Transparent when "Too Deep"

When I split header bars with a separator, sometimes the separator is transparent, leading to

This works (screenshot below):

- window title
  - box
    - header bar
    - separator
    - header bar

Separator's background-color CSS property: rgb(182,182,179).

But this doesn't (screenshot below):

- window title
  - box
    - box
      - header bar
      - separator
      - header bar

Separator's background-color CSS property: rgba(0,0,0,0.1). It's transparent and shows the black background below.

Unfortunately in my UIs I need to go that deep and I need to properly separate the header bars. :/