Epilog:
"Brian" and "Blogbeard" both had the right direction in suggesting a hidden resource file.
Early in the execution of the code lay a method call to an obscure subsystem that no one knew anything about. That subsystem turned out to be a defunct language translation module which was supposed to translate all of the labels in a form from a native language to a target language.
It was configured to keep the original labels and their translations in a file outside the scope of the IDE. Changes in the form itself were completely ignored if the translator found the label name in its file.
Moving the form contents to another package effectively disabled the translator.
The elegant solution turned out to a three-line change that removed the method call to the translator. (it was no longer required in the application)