Attributes of my problem: IF after development, you capture the negative and take it into post processing (Capture One) and aren't careful, you begin to see effects where it looks as though there is a picture in a picture - as if there is a square vignette but in reverse where the center received more development than the edges of the frame. This is almost as though there were a mask on the film! and calls for a lot more investigation of the gear, negative, etc. All in, since there is no such thing, something ain't right - I"m just not sure what, and in classic form, figured the likely culprit is me.
BVY: Your question is a good one. I like your implemented "hack". You should tell more about your hack, 'cause it might be very useful. Thanks!
FWIW, I'd actually watched that youtube. Have to say that I'm increasingly coming to believe that the key to agitation - and perhaps more noticable with 120 than 35 where film width is smaller - is the verticle inversion. Your "hack" addresses this directly. Rolling agitation with 120 using my existing equipment may have less internal rolling than the drum exterior shows, and therefore could result in the film sitting in the bath more than it appears. I tried to cure this with less chemistry in the container.
Matt has cured it by dropping machine agitation from the Developer step, and I'm tempted to do the same. But still have this project in mind to find a resolution that works... on the cheap. Love to find one that would work both for 35mm and 120 and multiple reels of film, but not there yet. Gonna have to do a lot more study to see what I really need to do.