From the documentation, findHighlightForeground (or find_highlight_foreground in the new format) are supposed to “override the scope-based text color”. This works fine if the key is present, but if I remove it, the text is black instead of the expected scope-based text color which the key no longer overrides.
I.e. I was expecting the text to be syntax-highlighted and only the background to change when that key is not present, like the corresponding key for selection works (selectionForeground).