Stave content ends up on different staff after splitting

a very strange thing happens when I try to split a flow: the music ends up on different staves.
This is Flow 54 of a 58 flow project.

I’d like to split Flow 54 into smaller chunks - here at rehearsal mark [C]:

So the bassoon stave ends up at violin 2 and the violin 1 stave ends up at the bass line:

If I extract that Flow 54, and try the same procedure, the problem isn’t there any more.

After exporting the flow - splitting:

This certainly sounds like a bug, and not a very fun one to look at. If you want us to investigate, you can provide us with the whole project.

I’ll posted the project file in as PM.