Ah OK, that explains it, thanks (once I parse “concurrent notes on opposing sides of a beam”). If I understand correctly then it’s more about the stem than the beam. What the chord in bar 5 would need is a stem going down from the left side of the C/G noteheads and connecting to the B on the staff below on the right-side of the notehead. So the B would actually be to the left of the C/G, as is quite normal in chords with intervals of a second. Not impossible then, surely?
I’m finding the mixture of beams between the staves and beams above the staves a little ugly, as well as more difficult to read than if there was consistency. Does anyone know of a way to force all cross-staff beams to be above the upper staff?