Acronyms don't have to be JUST initials; initialisms may not be other letters other than the first. Acronym is the broader term.
So the acronym "STFU" is also an initialism. But the acronym RADAR isn't an initialism.
I have never heard (or even thought about it) that an acronym has to be "pronounceable" as a word.
I'm sure there is some term for acronyms and even initialisms which ARE pronounceable, but I don't know what THAT is.

I have never heard this in USA. It sounds wrong to my ear.

I feel ya.
I DO think they mean slightly different things, although end up meaning the same thing, anyway.
One is "I have an obligation to work" ; the other is "I have an obligation tomorrow"