Why isn't MUSICAL SYMBOL NULL NOTEHEAD default ignorable?

Mon Sep 5 19:34:38 CDT 2016

It has just come to my attention that U+1D159 MUSICAL SYMBOL NULL NOTEHEAD
is not default ignorable, even though it has no visible glyph appearance
and no advance width in text, just like the various Hangul jamo fillers
that *are* default ignorable. Is there a technical reason for this or is it
just an oversight?
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