I don't see the filter ring making any difference,on or off, as you will see it screws in to cover the same depth of chrome as on the lens, and is chrome as well which could cause reflections but wouldn't be like your images.
I have never tried a 67mm fitting but all my references state the thread, probably the pitch, is not that of a filter thread, if the fit is snug that's obviously fine.
I asked about the sensor because some can respond to light strangely and this lens is retrofocus in design but if it's on film, then it is the lens.
I went out and took some really boring shots but couldn't provoke the effect, but then despite the coatings on this lens, I find it hard to provoke flare anyway, and this doesn't look like flare.
Final throw of my dice: what stop are you using, I'm wondering, given the uniform shape, if this is not simply excessive vignetting for some reason, so if you adjusted exposure on the centre you would see the edges dark, Phocus can apply negative vignetting which may "fix" it. As to why I am at a loss, my example does not do that, wide open there is some fall off but not enough to be troublesome, a shame as the old lenses do render beautifully in the right circumstances.