Atollic's TrueSTUDIO IDE / SDK displays unbelievable variable values
It turns out that a watch dog timer was active even after TrueSTUDIO stopped on a break point.  (I noticed several LED indicators on our target board continued to flash even after code execution stopped.)  I assume the watch dog initializes USB CDC values and thus caused the expected data to be replaced by "unbelievable" data.  Once the call to start the watch dog timer was commented out, the USB CDC variables contained the expected data.

I suspect that STMStudio reported the correct USB CDC values as code was executing and the watch dog was therefor not activated.  

Thanks for everyone's input!