The RhythmicStaff is not squashing out all the noteheads like it should be.
It is common to want to re-use the notes of a chord progression as either a
rhythmic staff or a notes staff (to avoid having to write more music just
for the rhythmic staff). When you use chords on RhythmicStaff it is not
squashing out the noteheads. IOW RhythmicStaff should only recognize
durations and not care about notes. Here is sample code that demonstrates
the bug:
\new RhythmicStaff {
\time 4/4
<c e g>2
<c e g>2
<c e g>2
<c e g>4
<c e g>4
}
\new RhythmicStaff {
\time 4/4
<c>2
<e>2
<g>2
<c>4
<e>4
}