debugPropertySet not triggering

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Clicking on the button "multi-instrumental" on the page https://github.com/mudcube/MIDI.js/blob/abcjs/examples/MIDIPlayer-v2.html (after the bugfix mentioned below) changes the global variable MIDI.Soundfont, but debugPropertySet(MIDI, "Soundfont") doesn't trigger a breakpoint. Neither does debugPropertySet(MIDI, "Soundfont", "trace") print a trace. Am I using it wrong?

The bugfix is to provide valid URLs to a MIDI file and SoundFont, i.e. to replace
<button onclick="load()">multi-instrumental</button>
by
<button onclick="load('https://bitmidi.com/uploads/107090.mid')">multi-instrumental</button>

and to replace
MIDI.setup({
by
MIDI.setup({soundfontUrl: "https://gleitz.github.io/midi-js-soundfonts/MusyngKite/",