[wfme]

It depends how you pronounce "H". If you pronounce it aitch instead of haitch then using "an" in this context is totally correct. https://blog.apastyle.org/apastyle/2012/04/using-a-or-an-wit....

[jaimex2]

You're right! Wow, I never realised rules get changed by pronunciation before. It's not vowels at all but whatever sounds like one in the writers mind.