Unable to programmatically undo/remove readonly property on fields

Given I have code like the following in Document Javascript:

function ReadOnly() {
  var theOption = this.getField("option5");
  var theField = this.getField("text57");
  theField.readonly = !(theOption.value && theOption.value == "Choice1");
}
ReadOnly();

This code will successfully set fields to being read-only, but the opposite doesn't work.  It's as if setting readonly to false doesn't have any effect.

Just to make sure, I tried variants of this, such as the following, but it doesn't change the fact that setting a field's readonly property back to false doesn't have any effect. The field stays read-only once the property has been set to true.

function ReadOnly() {   
   var theOption = this.getField("option5");
   var theField = this.getField("text57");
   if (theOption.value && theOption.value == "Choix1") {
     theField.readonly = false;
   } else {
     theField.readonly = true;
   }
} 
ReadOnly();