The full score looks like this:
The condensed score looks like this:
What went wrong in the flute parts and how do I fix it?
There must be a manual condensing change somewhere before this measure which is forcing the flutes into a single voice against their will. Put another manual condensing change on bar 426, check the Flutes checkbox, scroll down and reset:
If that doesn’t fix it, we’ll need to see (a cut-down version of) the project file to investigate further.
Thanks, @hrnbouma I deleted that condensed score, in which I had been experimenting for the first time with the manual condensing options, and generated a fresh score that had no changes and the problem vanished. So as you said, I had somehow introduced a change previously that was affecting this measure.