“i” pronounced as “u”

I’ve noticed more and more that romaji “i” ([i]) sometimes gets pronounced as “u” ([ɨ]), and was wondering how widespread that is.

First I learned about how in the north of Japan speakers merge じ and ず. Then I noticed in speakers in some videos saying “-masuta” instead of “-mashita” (with the normal vowel devoicing/deletion).

Then today, I came across a video where a speaker said

kurishusa for 苦しさ
shu for 死
shikumu for 仕組み

“Shikumu” was especially surprising because before that I’d only heard “i” being replaced by “u” after sibilants.

Is this a known phenomenon beyond the じ-ず merger? Where does it happen? I haven’t heard about this vowel merger happening after other consonants, and I haven’t been able to find anything more on it. It’s definitely seems limited, but I’m curious to know when and where it occurs.