Unexpected GLR parse result

[gafusss]

Consider the following example:

from parglare import Grammar, GLRParser

grammar = r"""
DOC: S? QZ;

terminals

QZ: /Q?Z/;
S: /(Q|Z)/;
"""

g = Grammar.from_string(grammar)
parser = GLRParser(g, debug=True, error_recovery=False)

result = parser.parse("QZ")

print("Result = ", result.to_str())

With parglare==0.21.0, the output is as follows:

1:  DOC[0->2]
    S_opt[0->0]
    QZ[1->2, "Z"]
2:  DOC[0->2]
    S_opt[0->1]
      S[0->1, "Q"]
    QZ[1->2, "Z"]

Notice that in the first tree, the "Q" from the input is missing. However, as far as I understand, that should not be possible.
What am I doing wrong, is there a gap in my understating of GLR parsers?

[igordejanovic]

Your understanding is correct. This is clearly a bug. Thanks for the report.