GABC converter: inconsistently generating accidentals

[igneus]

The gabc language supports flat/natural/sharp accidentals for any note.

from music21 import converter
import chant21

chant = converter.parse('gabc: (c4) la(ixi exe gxg fxf dxd)')
chant.show('text')

The gabc converter generates a Flat element for any flat accidental encountered, but while for B and E it also produces a flat Accidental for the Note(s) affected, it doesn't do so for other pitches:

{0.0} <chant21.chant.Section>
    {0.0} <chant21.chant.Word>
        {0.0} <chant21.chant.Syllable>
            {0.0} <chant21.chant.Clef>
    {0.0} <chant21.chant.Word>
        {0.0} <chant21.chant.Syllable lyrics=la>
            {0.0} <chant21.chant.Flat>
            {0.0} <chant21.chant.Neume>
                {0.0} <chant21.chant.Note B->
            {1.0} <chant21.chant.Flat>
            {1.0} <chant21.chant.Neume>
                {0.0} <chant21.chant.Note E->
            {2.0} <chant21.chant.Flat>
            {2.0} <chant21.chant.Neume>
                {0.0} <chant21.chant.Note G>
            {3.0} <chant21.chant.Flat>
            {3.0} <chant21.chant.Neume>
                {0.0} <chant21.chant.Note F>
            {4.0} <chant21.chant.Flat>
            {4.0} <chant21.chant.Neume>
                {0.0} <chant21.chant.Note D>