StaffGroup \consist BarNumber with RemoveEmptyStaves
So if you \consists the "Bar_number_engraver" in the StaffGroup
context and remove it from the score context, it works fine and as
expected printing BarNumbers on the top of every StaffGroup.
But if you also \RemoveEmptyStaves (provided some StaffGroup are
flagged as empty) the now orphan BarNumbers bubble up to the top of
the score. This can goes unnoticed as they're often perfectly aligned.
Same goes for \mark and other similar grob.
Sorry for the crude MWE below but it clearly shows the issue.