Forms are not submitted if they are part of an amp-live-list once the live list has updated.

How do we reproduce the issue?

1. Open https://amp-live-list-form.gomix.me/ source
2. Enter something into the form and press submit -> input is printed inside the doc
3. Wait until the time updates via amp-live-list (15s)
4. Enter something into the form and press submit -> the form is not submitted

@mkhatib @erwinmombay