Weird behavior in nabc rendering

[rpspringuel]

Debug messages are, by design, not supposed to affect the typesetting output. However, in playing around with a new debugging feature, I've found a message (or two) which seem to be changing the way an nabc glyph is being rendered.

Take a look at the tex-output/syllable-rewriting test in the test repository. Comment out or delete the syllablerewriting debug messages in \gre@emit@syllabletext (either, or both) in gregoriotex-syllable.tex and you'll find that the test run against this modified file fails because of a small tick mark in the nabc glyph over the syllable "spi" in the first line (all three times) is moved to the left (I advise upping the DPI to 300 in order to make the changing pixels more numerous and opaque, but the algorithm detects the change at my usual 150dpi, the pixels just happen to be fairly transparent, and thus hard to spot). Normally I'd associate this kind of shift with a spurious space or missing %, but everything seems fine here in that regard.

Not knowing anything about the nabc, I can't tell which position of the tick mark is right.

Can others please take a look at this test and see if this behavior is reproducible on their setup.