GABC converter: flat/natural breaks a neume in two

[igneus]

Today I stumbled upon the fact that when parsing gabc, a flat (as well as a natural) in the middle of a neume is not considered element of the neume, but an "extra-neumatic" element, thus breaking the neume in two:

from music21 import converter
import chant21

chant = converter.parse('gabc: (c4) la(igixig)')
chant.show('text')

Result I would expect:

{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.Neume>
                {0.0} <chant21.chant.Note B>
                {1.0} <chant21.chant.Note G>
                {?.0} <chant21.chant.Flat>
                {?.0} <chant21.chant.Note B->
                {?.0} <chant21.chant.Note G>

Actual result:

{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.Neume>
                {0.0} <chant21.chant.Note B>
                {1.0} <chant21.chant.Note G>
            {2.0} <chant21.chant.Flat>
            {2.0} <chant21.chant.Neume>
                {0.0} <chant21.chant.Note B->
                {1.0} <chant21.chant.Note G>

Volpiano parsing behaves differently - flat is considered part of the neume here. Is there a bug in the gabc parser?

from music21 import converter
import chant21

chant = converter.parse('cantus: 1---jhijh')
chant.show('text')

{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>
            {0.0} <chant21.chant.Neume>
                {0.0} <chant21.chant.Note B>
                {1.0} <chant21.chant.Note A>
                {2.0} <chant21.chant.Flat>
                {2.0} <chant21.chant.Note B->
                {3.0} <chant21.chant.Note A>