In "Folder Margin Style" (Settings->Preferences->Editing), whenever any of the radio buttons are clicked, the Foreground colour for "Fold active" in Global Styles is set to white. This only seems to affect the current session, as restarting the program will fix it.
I noticed in "npp.5.9.src/PowerEditor/src/stylers.model.xml" that the line for "Fold active" under Global Styles is missing. Specifically, it's missing the line that says:
<WidgetStyle name="Fold active" styleID="0" fgColor="FF0000"/>
This line appears in the installed file, but not in the source. I'm not sure if this is the cause of the bug, since I was unable to compile and test it myself.