Sean Beckett's blog
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"Does anyone know why a Selenium click event might not trigger the same activity as directly triggering the DOM id through javascript? We have a form submit button that works fine when directly activated but doesn't work in Selenium."
It was suggested that perhaps this is a timing issue. Maybe some required JS for the form hasn't loaded before Selenium is trying the event.
One workaround would be to test only the form submission called by on-click instead of the click itself.