Just listened to Yoasobi new song: セブンティーン and realized they are using ブン (-bun) for the sound -ven instead of ベン (-ben), but are pronouncing seventeen as in English (not sebunte-in as the katakana suggests).
Is there a specific reason for that?
Apparently 7-eleven is written with ブン as well, is it pronounced sebun-elebun, or is it special cased?
It’s not special, it's just that katakana readings of foreign words are not for our (English speakers) benefit, they are for Japanese people. So it's not really wrong or "special."
It could be for a number of reasons, but my guess would be that for the word "seven", there is no way to accurately reproduce it in Japanese. In that sense, neither セベン nor セブン are accurate from our perspective. Most v sounds tend to be written with the ブ sound, so it's probably that convention taking precedence in this case :)
I would like to add that the second syllable of English ‘seven’ is not pronounced with any /e/-like sound in English. Depending on the variety of English in question, it either has an indistinct weak vowel /ə/ or is pronounced with a syllabic /n̩/ (or even /m̩/) without separate vowel at all. Identical situation happens in オーブン oven or ダブル double. I think it's just the closest match within Japanese phonology rather than being purely conventional, and indeed it can sound pretty close to original with some devoicing in the right place.
