I've just noticed something peculiar. I created a dynamic content user control with some dropdowns in it that have AutoPostBack set to true. If I include this dynamic content on a page and select a new value in a dropdown, a postback occurs but the OnSelectedIndexChanged event won't fire.
However, if I insert the same user control in the page template directly (as opposed to through dynamic content) the event wiring works like intended and the event handler fires properly.
Is something in the parent hierarchy DataBinding and if so: does it databind on postbacks as well as the initial request? If not I would guess that it might have to to with the page life cycle and when the dynamic content is populated. If you can't solve it , please register a bug with support and we'll have to investigate it more.