We discovered that the text on the contours are turned white (and thus invisible when printing) when you use Override Symbology on a referenced terrain model:
I have created a video showing what is happening, but it is too large to post. Let me know if I can Email it to someone.
The problem may be solved by using level override for color on the reference or changing the Text Style, but that is not very elegant.
This must be a bug, right?