I used the code provided to create a sample and to reproduce the issue - but on my side RadComboBox works properly and I am able to get the selected text and value correctly. Please find the sample attached.
I do not know what the implementation of OnPreRender event handler that you have created is, but on my opinion your code is all right.
Sometimes a similar issue related to item selection can occur if there are RadComboBox items with same values populated in RadComboBox - you can find more details about this topic here
Please check if there are items with same values populated in the RadComboBox.
the Telerik team