Hello!
It is not so simply.. Maybe my description is wrong :o(
I put clock signal at encoder clock input. I have reduced resolution of the receiving routine to 13 bits, because I have expected stop bit.
When oscilloscope is connected to the outputs of encoder it shows high state or - after turning by 180 degree - low state of the outputs. There isn't any changing of state of the data lines. But, as you've said, some of bits should change during turning.
I have bought the second encoder. The result of working is the same.
My customer wants to use this encoder, but I don't like components like this. This is "top secret encoder". Yes, you can use it with Siemens' devices but try to use it in your own application... If you need some documents, you can get only wire connections and mechanical (also electrical) characteristics. But it is another story, I think.

Greetings,
Jacek