something like oyasumi punpun?
My favorite manjo not clue what absurdism is but read oyasumi anyway.
(it's so unpopular barely anyone knows that manjo) nearly as unknown as FMAB
Not too sure exactly what specific aspects of absurdism/existentialism you're looking for; they're broad enough that you could (incorrectly) twist many series into applying to them with enough mental hoops. I've heard people trying to inject existentialism in Black Lagoon of all things.
Mono no aware is more of a Japanese/Buddhist thing than flat-out existentialism, but they share some relevant similarities. Tsukumizu's manga and doujin probably go the closest to what you're looking for, but you could probably pick up any series with that tone or themes of transience (YKK, Nausicaa) and scratch your itch. Planetes has moments of introspection that touches on similar themes, especially in the latter half. It's a good read either way.