For example, the word hikikomori (recluse in Japan).
Some sources use "an hikikomori," like this entry in IMDb.
And this article in the BBC uses "a hikikomori."
Are both usages acceptable? Or only one is correct?
The consonant -n is added to the article 'a' exclusively for phonetic reasons: to avoid fusion of 'a' with a vowel in the beginning of the following word. That word is not necessarily the noun to which the article relates, it can be an adjective before that noun. So in your example 'an' is improperly used.
P. S. However, exceptions are possible. In case the speaker intentionally pronounces a foreign word with a vowel in the beginning (despite its English spelling and/or pronunciation), 'an' would fit that pronunciation.