A bug creating parts?

I have been putting together a carol book of local carols. Did them as separate files then cut and pasted them all together into one so they would print out on the same sheet.
I created a new section with a section and page break for each carol, very carefully re-setting the time and key signatures at the start of each section.
I came to try them out today (fortunately just with a quartet, not the whole band), and we found some errors in key signatures in the parts.
They look fine on the score, and have printed out differently to how they appear in the part still in the mcsz file.
Take the example of "Back Lane" that I have screenshotted.
On the paper copy, there is a sudden random key change on the second line in the part.
The score shows that line (here we're looking at Tune in Bb) in the correct key throughout.
The part in the file, now has the incorrect key signature and accidentals, which it didn't have when the part was extracted, as can be seen from the printed copy.

Any ideas what's gone on here?
How can we trust the programme to extract parts correctly without checking each individual note in each part - in which case it might be easier to do the whole thing by hand! Surely the extracted part should match the score?