Resolved! Error in Reference Manual - RM0440 Rev 4 - Page 358, section 9.3.13, "Using the HSE or LSE oscillator pins as GPIOs"
The word "OSC_IN" is missing from the following sentence:"When the oscillator is configured in a user external clock mode, only the pin is reserved for clock input and the OSC_OUT or OSC32_OUT pin can still be used as normal GPIO."Should say: "...onl...