I am not sure how an event handler connected to a specific DOM node could be fired off by clicking on other DOM nodes. Are you sure dojo.query always gives you the button's DOM node?
To make sure the event handler is connected to the right DOM node, it would be easier to use the reference to the button's DOM node you already have, instead of searching the entire DOM with dojo.query (by using this.connect the handler is automatically called with widget scope, and the connect is disconnected on destroy):
this.connect(this.dynamicButton.domNode, "click", this._OnSubmitMicroflow);
It would be even better to directly pass a click handler as parameter to the _Button's constructor (it would be great if there would be documentation for widgets such as _Button):
this.dynamicButton = new mendix.widget._Button({
caption : this.ButtonLabel,
action : dojo.hitch(this, "_OnSubmitMicroflow")
});
This way, the handler will also be called when the button has the focus and [enter] is pressed.
I should connect by instance dojo.query('#' + id of the widget) or directly to this.dynamicButton.
Edit (avoid mistakes): I should connect by instance dojo.byId(id of the widget) or directly to this.dynamicButton.