I'm playing with Orbeon 4.0.0.m7.201207162324 PE.
I copied the code from
http://wiki.orbeon.com/forms/how-to/fb-fr/form-builder-wizard#TOC-Get-the-source and pasted into Builder.
Then I clicked Save -> Publish -> Open Published Form New Page and the form was displayed but without the "Next" button.
I came back to Builder, edited the source and noticed that Builder removed the </fr:buttons> section.
This is 100% reproducible.
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Kind Regards,
Bartek Gawel