Gradebook > Adding comments sometimes appears not to work

I encountered this problem today while testing something unrelated. Unfortunately, I haven't found concrete steps to reproduce as sometimes it works and sometimes it doesn't.

Here's the actions I took:

1. set up an external gradebook item (assignments, samigo, etc.)
2. go to gradebook, add a comment for a student in the external gradebook item column
3. provide some text; Save
   • The comment I used was "Where's your work?", perhaps the apostrophe has something to do with the problem, it's repeated in the JavaScript error output (below)
4. Result (sometimes):
   1. The modal dialogue doesn't go away fully, you are left with the title and an 'x' (see screenshot 1)
   2. after manually closing the dialog, the respective cell in the gradebook doesn't receive the icon indicating there's a comment
   3. if you click add/edit comment again, you'll see your text
   4. if you open the browser's console output, you'll see the following JavaScript error:

Wicket.Ajax:  Wicket.Ajax.Call.processEvaluation: Exception evaluating javascript: SyntaxError: missing ) after argument list, text: (function(){GbGradeTable.saveNewPrediction('Where's your work?');})();(function(){var win;
try {
	win = window.parent.Wicket.Window;
} catch (ignore) {
}
if (typeof(win) == "undefined" || typeof(win.current) == "undefined") {
  try {
     win = window.Wicket.Window;
  } catch (ignore) {
  }
}
if (win && win.current) {
 var close = function(w) { w.setTimeout(function() {
		win.current.close();
	}, 0);  };
	try { close(window.parent); } catch (ignore) { close(window); }
}})();

If you refresh the gradebook, the icon in the cell appears as it should.

Sometimes it works properly, sometimes it doesn't. I haven't identified what the root cause is, and I'm not sure if this affects 19.x as well.