This same thing happens with "MVP" I think. "An MVP" sounds good, but "An Minimal Viable Product" doesn't. So I use a/an depending on whether using the abbreviation or not, but I have no idea if there's a rule for this or not.
In your example "MVP", the first syllable starts with an "em", so a vowel. In "Minimal Viable Product" the first syllable starts with the consonant sound.
I had a major pain with this rule when dealing with auto-generation of some API documentation. It ended up just being an issue of detecting/guessing if the text was an abbreviation or some weird CamelCase thing.