When entering a JSON body using the iOS App, make sure that "Smart Punctuation" on your keyboard is OFF.

Otherwise the quotation marks sent will UTF-8 "left double quote" and "right double quote", not an ASCII 0x22 as JSON requires. Thus, the JSON body will be invalid and rejected by the server.

Settings > General > Keyboard, and then turn off “Smart Punctuation”

Lost a few (more) hairs figuring this one out. Hope if saves someone else from a life of Rogain.