Here's my thinking over the last couple of days. First, I'm attaching a screenshot of the relevant portion of checkfonts.xml. You can see that there are multiple versions of the font installed. Bizarre.
I just uninstalled Scribus and then manually deleted the Scribus directory from Appdata/Roaming. Then I reinstalled, Scribus recreated the font cache, and it looks exactly the same as before (with the multiple font instances). Even more bizarre. I can think of only two explanations for this.
1. Scribus itself is somehow storing font information in a way that survives a complete reinstall with the old checkfonts.xml removed. This seems very unlikely.
2. Windows stores font data somehow when fonts are edited or removed. This sounds weird but I have had intimations in the past that Windows does funny things behind the scenes with fonts. For instance, a couple of times it has renamed font files when I reinstall them. In the Windows/Fonts folder all one sees is the most recent version, though.
Also, I do not believe all the font versions seen in the screen shot were ever installed in Scribus. I experimented a bit with 1.5.2 about a year ago, then in the last 2-3 weeks have been playing with 1.5.3 svn. If Scribus was remembering the font data, the xml file would show one or versions from a year ago and a couple from recent weeks, but not all the intervening ones. This points (as far as I can figure out) to a Windows problem, perhaps made worse by a bug in Scribus' font handling.
I was trying to test the OT features in my font and found that Scribus behaved differently on Win 7, Win 10, and OS X. It now seems that the immediate cause is that Scribus was using outdated font versions on Windows (different on the two machines), so it's no surprise that that the font didn't work as expected. (The oldest versions don't have the OT features at all, and more recent versions were experimental in terms of how I set up the features.) Scribus and the font play nicely with OS X, where (I think) it is actually using the current font version. The underlying cause is perhaps a bug in Scribus. Does that make sense? I have never had this kind of problem with any program other than Scribus, and I have done a lot reinstalling fonts as I edit them