MetronomeMark and break-visibility

[Thomas Morley]

Hi,

the following code for _Rehearsal_Mark works as expected, i.e. the
RehearsalMark is printed at end of line:

{
  b1
  \override Score.RehearsalMark.break-visibility = ##(#t #f #f)
  \mark \default
  \break
  b
}

Alas, the same for _Metronome_Mark fails, the MetronomeMark is not
printed at all:

{
  b1
  \override Score.MetronomeMark.break-visibility = ##(#t #f #f)
  \tempo "Andante" 4 = 60
  \break
  b
}

although MetronomeMark has settings for break-visibility.

Is this on purpose or a bug?
How to make it work?

Thanks,
  Harm