It would figure, I always do end up trying to do things weird.
Totally have the same problem with your file on 1.5.1, which is what I'm using, so that seems to be the problem.
Updated from 1.5.1 to 1.5.3 and ...the display issue isn't a problem. I can only offset so far in that version, though (even your file shows up with the down-shifted text basically underlined; the vertical shift only really moves the strikeout downwards).
1.4.6, the last 'stable' version, works just fine for the strikeout/shift, so I'm making a fresh document from scratch (my publisher's preferred template doesn't load in it since it's from a newer version of Scribus, naturally).
Thanks for helping me out here!