Inconsistent presence of separator line on new-tab page in private mode

Steps to reproduce:

Close all Orion windows.
Open a new Private Browsing window.
Notice there is a grey lower separator line below the "Private Browsing Enabled" banner.
Navigate to a website in this window (via the URL bar)
Open a new Private Browsing window.
Notice there is no divider line below the "Private Browsing Enabled" banner

Image/Video:
Top window is as expected, lower window has the incorrect behaviour

There is a very thin and light line on the second window. It just isn't the correct thickness and color.

This only happens in light mode, oddly. Toggle dark mode and the line is immediately visible.