I have worked with the presentation and code snippet shared by you. The issue is that you are setting text for the shape directly on textframe level. When you will do so, all the inherent properties of the text formatting in that text frame will get reset. There are certain font related properties that are set on paragraphs level and some at portion levels. In order to retain all the text related properties in the cloned slide shape, the text must be replaced on portion level. I have made the following correction in your code snippet and it works fine. Please share, if you still face any issue.
tf.getParagraphs().get(0).getPortions().get(0).setText("textframe - the formatting is wrong");
Thanks and Regards,
Never mind - the copy constructors seem to preserve formatting. Thanks again for the quick response, appreciate it.