Architect- was that PB5 you were reading 3V3 on? Pin was set to what state?
I found the document explaining the resistors (inluding pb5). These resistors determine the bootloader interface. It is detailed on page 51 of this pdf document: http://www.stmcu.org/download/index.php?act=down&id=942
I think that pulling up/down specific resistors on bootup determines the bootloader interface. Although, after that, I’m not sure that the resistance is still necessary.
I’m chalking this one up to damaged/defective hardware. Marking Gus’s response #9 as the answer.