Good old Finale, with which some of these files were also encoded (Finale d seems to ring a bell?), back when I wanted to convert a lot of Encore files into Sibelius (don’t know about this one) back in the day, when it was the only (convoluted) route. The peculiar tuplets are a surprise (the original has none in this section) but it’s at least appearing, and much of the original notation is reproduced. Daniel has already commented on the original music xml specification being written by the same person (?) who worked on Finale.
Thanks for adding to our fund of knowledge here, Rob. I can’t help feel that different notation programmes’ handling of grace-notes (some later on in this piece) might be what’s hanging up conversions, not to mention slurs, which early Finale and Encore gave wildly different but equally ennoying results with. Oddly, some more recent music xml bundled with the latest version of Encore (not really a ‘version’ so much as the old programme modified to run on modern compiters) does convert but in both Sibelius and Dorico shows peculiar artifacts of timing and spacing. I’ve also been unable to open music xml from the iOS version of notion, but a friend has had good results importing from the OSX version of Notion.
(I work in a library attached to a university music school, so I have access to people with lots of different approaches, and the appropriate software, as well as what I’ve bought over the years, in case anyone’s wondering—but I haven’t used Finale—the first notation software I used, on Macs—since around 1991). (Sorry to be long-winded: worse than members being grumpy?)