Garry, thank you again for your efforts.
As far as I personally am concerned, there's no issue (as far as I can tell). I started out wanting to test the OT features in Scribus and got different results on Win 7, Win 10, and Mac. That made me think there was something wrong in Scribus' handling of OT or possibly with my font. After uncovering this weird behavior with the font cache and testing with a renamed font, I'm satisfied that the OT features are working correctly, as is my font. That's all I need to know.
While most people don't reinstall the same font over and over like I do when developing a font, it certainly can happen (as when an updated version is released). So it would probably be good to get this problem fixed. I can file a bug report, unless let's better to proceed in some other way.