No ID for acoustic guitar in Instruments.xml
S4 - Minor
Starting on line 9944? of instruments.xml here is the definition for the Acoustic Guitar
<Instrument id= > <longName>Acoustic Guitar</longName> <shortName>Guit.</shortName> <description>Acoustic Steel-strung Guitar</description> <musicXMLid>pluck.guitar.acoustic</musicXMLid>
Notice there is no ID assigned to it. I would suggest that "Acoustic Guitar" be assigned. I may have a slightly old version of Instruments.xml loaded so the line number might be different but the ID is missing. The problem with this is that it makes the instrument unavailable to plugins.
Whatever version you have, it doesn't match the current one, see https://github.com/musescore/MuseScore/blob/master/share/instruments/in… or https://github.com/musescore/MuseScore/blob/3.x/share/instruments/instr…
In MuseScore 3.5 plugin API has methods which operate with MusicXML Sound ID of the instrument, like
There is a limited number of such IDs, and MuseScore often has several instruments using the same MusicXML ID, so it is not always possible to uniquely identify an instrument by such ID. However MusicXML IDs are better standardized and may be more convenient to use in some cases. Maybe this approach may be used as a workaround in your case as well.