Plugin API: annotations in MusicXML file have blank text

1. launch MuseScore 3.4
2. open attached file "Amazing Grace.musicxml"
3. open the Plugin Creator and load the attached plugin "test5.qml". This is a tiny plugin that finds all the Harmony annotations and prints them to the console.
4. run the plugin

Actual Result: all the Harmony annotations have blank text. The console says:
Debug: 1440
Debug: 2880
Debug: 4320
... etc

Expected Result: the Harmony annotations have non-blank text. The console says:
Debug: 1440 G
Debug: 2880 Bm
Debug: 4320 C
... etc

Workaround: after opening the MusicXML file, save it as an MSCZ file, then close it and re-open it. Now the plugin gives the expected result.

OS: macOS 10.15, Arch.: x86_64, MuseScore version (64-bit): 3.4.2.25137, revision: 148e43f