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Using on Canon MkII, I've found that my recently bought, little used Zuiko 21mm F2 has vignetting which never completely goes away with stopping down (no filters on!), although it reduces in size it gets sharper and more noticeable
My copy didn't do that on film - I checked some original slides. This was one of my favourite lenses on film and I have lots of those.
It is possible that the angle of incidence at the corners is enough to cause vignetting on Canon sensors but not film (the problem Leica had to solve with their angled microlenses). This may be one of the lenses Olympus was referring to when they said their OM lens designs were not "telecentric" enough to build a digital OM body. On the other hand, I have located some test shots I made on my 5D with the same copy of the 21mm f/2 I used for film and they look OK. Unfortunately the notes I made were a bit scrappy and I don't totally trust what apertures they were shot at. There is an example there with no vignetting marked as f/11 - for sure it's gone when fully stopped down. I decided to go with a Canon zoom, no longer have this lens, and never did more with it on digital than test it, so can't help you more there.
Otherwise, it could be a bad copy or maybe, as suggested, your adaptor.