I *think* this has to do with control focus. Normally, the desired result when a user clicks the 'Enter' key, is that a form automatically submits but I have ran into the same problem a few times.
My work around was a JavaScript that looked for the enter key to be pressed and just called [form].submit(); to post the form, not sure if that will work for what you are trying to do. (Obviously calling [form].sumbit() is just to easy, you will need to actually raise the buttons click event handler to do any processing on the server inside that event handler)
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