Excellent explanation @gr5. NOW I get it (I hope), even though I have read it on many places previously including comments from you. Thank you!
After inspecting previous calibration cubes I saw the rounded corners has been there all the time and as apparent. My test changes to acceleration and jerk did nothing upon closer inspection (might been to small changes). And it's mostly print using AA0.4 at .2mm layer height that have the issue. AA0.4 at 0.15mm looks a lot better, AA0.25 is more or less flawless and AA0.8 at 0.4mm have good corners. All prints using defaults for UM3 and UM PLA.
Slowing things down is not an issue for me, I'm almost never in a rush with my prints.