Yeah, that’s from the late Middle Chinese, mainly from after Song and Ming dynasties. There are three sound shifts before certain vowels, usually /u/, /o/ or /i/:
That’s what you are hearing in Hakka, but it has nothing to do with the /w/ → /v/ sound change in Mandarin. Also, as previously mentioned, the small percentage of Hakka speakers is unlikely to attribute to a large number of English learnings doing the “fwu” thing.
Regarding Hansioux’s Q on “which” and “when”, I pronounce them as “wich” (sounds like “witch”) and “wen”. No real “h” sound in my accent. I also pronounce “what” “where” “whale” as “wut” and “ware” and “wail.” Born and raised in the U.S. Northeast and but have lived in California now for almost 15 years.