Source Text Reverting to previous value

It seems to be a bug. You need to use a script to fix it.

Run the script below.

It should set the selected text layer value with the expression value and disable the expression.

You can also directly add the composition to the render queue.

CSV_FIX.jsx

var proj = app.project;
var thisComp = proj.activeItem;
var selectedLayers = thisComp.selectedLayers;
app.beginUndoGroup("undo");
for (var i = 0; i < selectedLayers.length; i++) {
  if (thisComp.selectedLayers[i] instanceof TextLayer) {
    var sourceText = thisComp.selectedLayers[i].text.sourceText;
    sourceText.setValue(sourceText.value);
    sourceText.expressionEnabled = false;
  }
}
app.endUndoGroup();

// AddtoRenderQueue
app.executeCommand(2161);

// OR using Media Encoder
// app.executeCommand(3800);