Summary says it all. Steps to reproduce:

1. Create a job
2. Add a validating string parameter with arbitrary data
3. Save the config
4. From the "Manage Jenkins" page, reload the configuration from disk
5. Go back to the job config and see that the validating string parameter is gone!

I discovered this trying to load configuration programmatically from xml. Initially I assumed there was some error in my xml, but alas the scenario fails even with brand-spanking-new-generated-by-the-plugin xml.