@Jack Ketch Re: Pronunciation comes from the words it acronymizes
"I keep seeing this 'rule' being parroted, but can anyone actually find a source for it?" Yes, it's called 'derivation'. Look that up in a dictionary. A good dictionary will even give you the derivation of 'derivation'.
"I know of many acronyms in regular use which do not follow it" So do I , but they all should.
"and there is no such controversy over their pronunciations." There is plenty of controversy: see commentards above and below.
"Acronyms are words in their own right." They can become so, given enough use, though usually acronyms are just acronyms not words.