As you can see I’m using hidden tuplets to make notes cross barlines, reflecting the original notation from 1591. When I put a system break on bar 6, something about the fact that the 1 is ‘empty’ results in this curious orphaned staff. Adding another system break at the start and telling it to wait for the next one fixes it easily, but I haven’t seen this before so I thought I’d share. 
Two questions before I dare a guess:
- in Preferences, have you disabled the option to snap breaks to existing barlines?
- when you created the system break, what was selected? – the barline tick or one of the notes with which the music in that bar starts (i.e., one of the notes after the ‘empty’ position)
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Well spotted on both fronts. Turns out it was just user error instead of a fun new bug…
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