OpenType features inconsistent between Win7 and Win10

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First, let me thank the Scribus team for adding support for OpenType features.  I've been following Scribus for some time but have never been able to use it because my work frequently requires support from OT features.

I have created a font that that supports, among other things, the Old Italic block of Unicode.  Under the Old Italic script, default language, I have three stylistic sets and the mark to base positioning feature.  The font works correctly in XeTeX and Word 2016.

With Scribus 1.5.3 svn on a Windows 7 machine, the stylistic sets work correctly but the mark to base positioning is ignored.  Under Windows 10 (with the recent Creator update) the behavior is reversed: combining underdots appear exactly as they should, but the stylistic sets do not operate.  In Text Properties/Font Features, I see the checkboxes for the three stylistic sets and I can place the checkmarks, but the characters on screen do not change.  This is with the exact same font file on both machines.

The OT features for Latin script that I've tested so far, including stylistic sets and mark to base positioning, all work on both versions of Windows.  So I suspect that this issue has to do with scripts that are in the supplementary planes of Unicode.

I am happy to file a bug report and can supply screen shots if needed.  But since I am new to Scribus I thought I'd post here first and see if anyone has comments/suggestions.  Downright weird behavior!