That's just weird over-correction. "A historic day" is correct but for some reason people heard "An historic day" and the idea spread that it was a special exception.
Kind of like how you see lots of people on Reddit saying "water isn't wet, it just makes other things wet". They heard someone else say it and it sounds smart so they repeat it, without spending 10 seconds to look it up in a dictionary.
To me it may be a tell for the writers at one point pronouncing it as "'istoric", similar to those who say human as "'uman", and then as you said likely fanning out as an over-correction, possibly because that dialect was associated with prestige.
Kind of like how you see lots of people on Reddit saying "water isn't wet, it just makes other things wet". They heard someone else say it and it sounds smart so they repeat it, without spending 10 seconds to look it up in a dictionary.