Just found the root cause for this and here's the solution/explanation in case someone bumps here:
Just check ZF-11953 on Zend's Issue Tracker
http://framework.zend.com/issues/browse/ZF-11953A comment on that ticket:
MediaXtend wrote
The problem is that when Dojo parse page's Dijits, it sometimes adds the required="true" or required="false" attribute on some of the form's elements.
Since this attribute doesn't exists in HTML4 nor in XHTML1, new browsers (like Firefox or Chrome) is using the attribute in its HTML5 form, discarding the "true" or "false" value. So the form needs to have all the Dijits with the attribute "required" to be filled to be submitted.