Jason I agree with your speculation and Peters comments.
brew time for espresso with fines is seconds
brew time for press with coarse is minutes
so naturally if you have some very coarse or even whole bean in your portafilter, when you pull your shot it is under extracting, or unable to get sufficient TDS for this brew method from the coarse/whole bean, while possibly over extracting the remainder as a result.
so with a press the more unwanted fines you have, the more bitterness. Not bitter flavors you might find in a dark roast, rather natural bitter flavors that are not sweet that shouldn't be extracted from bean. If you want to be clear on the impact few fines have in a press, make a whole press with 100% fines, exagerate the mistake! Some people call this complexity. I call it artificial body as these fine particles or sediment effect the flavor via texture not the taste. This is not something that is relevant to bean, rather brew method.
I had the opportunity to take a close look at this grinder last month http://www.malykke.dk
and I was quite critical about the percentage of fines, certainly more than I typically find out of a new kf804. But you're right Jason,.... there's always fines unless you sift. Time to go practice your speed sifting techniques.
Nice picture Mark!
Edwin D. Martinez