Currently, the "epub:" namespace is changed to "ns2:" in TTS generation. The declaration is still validly pointing to the correct namespace URI (http://www.idpf.org/2007/ops), but it is confusing to introduce a new term for that namespace, so the desired result is that the "epub:" name be kept intact in TTS generation.
Comment from @bertfrees:
We should be able to do something about this. Not sure where exactly the prefix is changed though. Anyway, the current result looks valid EPUB as far as I can tell, so this should not be an issue for reading systems.