History of 'acronym' versus 'initialism'?

Nowadays on the internet there’s a contingent who make a strong distinction between ‘acronyms’, which they say must be pronounced "phonetically" (for lack of a better term), and ‘initialisms’, which are pronounced as disconnected letters. — Call "FBI" or "TNT" an "acronym" online, and there will likely be someone who will pop up in the comments to "correct" you.

But from my understanding, this sort of distinction wasn’t/isn’t always the case. Indeed, the first attested use of "acronym" according to the OED is one of separated letters: "Pee-gee-enn. It’s an acronym [Ger. Akronym], that’s what it is. That’s what they call words made up of initials."

Current online dictionaries are somewhat equivocal about it. Wiktionary saying "… sometimes exclusively … when pronounced as a word" (with corresponding usage note). The OED has two definitions which cover each case, and no indication either is preferred/discouraged (though notes that it itself uses "initialism" for the pronounced-as-letters variety). Lexico only seems to recognize the "pronounced as a word" variant. Dictionary.com has both definitions, but does discuss the distinction with initialism in a usage note. Collins Dictionary has a number of sections, with pronunciation not being explicitly treated in the American English section, but with "a pronounceable name" being explicitly mentioned in the British English section. Interestingly, it also defines "initialism" in British English as "an acronym in which each letter is pronounced separately" (emphasis added). Merriam-Webster has a main definition which seems to imply the fluid pronunciation definitions, and treats the pronounced-as-letters definition as an "also" note. — None of the dictionaries, however, seem to treat how the distinction has been treated over time, aside from that implicit in the OED’s dated usage examples.

I don’t know if there’s any "historical" dictionaries online, but the one hard-copy dictionary I currently have access to is the 1997 Merriam-Webster Collegiate Dictionary (tenth edition), which has basically the same definition for "acronym" as the current online version (minus the "also" note, rendering it more ambiguous), but defines "initialism" as "an acronym formed from initial letters" (differing from the current online definition, which uses "abbreviation" instead). Google Books Ngrams seem to indicate both words took off starting in the 1960s, but with a greater preponderance of "acronym" to "initialism", though "initialism" seems to take a (relative) upswing in 2013. However, I’m not sure if there’s much you can infer from that.

All of which is a long-winded and somewhat pointless background to my question: How has the definitions and usage of "acronym" and "initialism" changed over time? Was the pronunciation distinction prevalent from the start? If not, where did that separation originate from, and how did it spread? (With a particular interest in the prevalence among the internet pedant-erati.)

Note: When analyzing definitions, please make the proper allowance for a distinction between being a word and being pronounced as a word. It does not follow a priori that when someone else says something is a word that they necessarily mean the same thing that you mean when you say it should be pronounced as a word. It might, but I’d say it’s a fallacy to take that as a given.