You beat me to this one! 
I spotted it yesterday and forgot about this post (and what the blank selects were supposed to have in them)
At the same time I discovered this one;
In my form, I have a question that has calculated values in the label and hint (and guidance), the label also had some static text, the hint did not.
The result in WF was:
Label: Some text [calculated label text]
Hint: --ODK-OUTPUT-STRING-0--``--ODK-OUTPUT-STRING-1--
Modifying the form so that the hint field was some text ${hint_text_calculate} then resulted in:
Label: Some text [calculated label text]
Hint: some text [calculated hint text][guidance text]
The issue appears to be putting a field in a hint with no other text. But a label with a field and nothing else is ok.
I also tested a field followed by text and that worked, resulting in
[calculated hint text]some text[guidance text]
I am assuming that string0 is the hint and string1 is the guidance note, as guidance isn't properly supported yet, but does appear.