Why dynamically created user controls disappear when controls are not doing full postbacks?

Based on my current understandings, when you have an UpdatePanel control there is no full postback. Therefore if I dynamically add custom user controls and they have UpdatePanels that are updated in their code they shouldnt disappear from the page they are loaded into, right? Apparently not. I made a simple project to test and still my dynamic controls disappear when clicked even though they should not be triggering a full postback. I have an aspx page that loads the controls:

Update Panel error: Control with the ID “xxx” could not be found in the UpdatePanel

I have a composite drop down calendar user control that consists of a textbox and and calendar image and a validation control. I expose a property called “TextBox” on the usercontrol which returns a reference to the textbox used within the control. This is the textbox that the user enters the date into.