So after months of testing, reading articles and so on, I can confirm Studio One still has serious midi jitter issues. What you play is not really what is being recorded.
I've run many tests, with other audio interfaces, DAWs, computers, equipment, versions of Studio One. I have even done some clean windows installations, but nothing seems to fix it, so I can only say that Studio One is at fault.
I have played drums and piano parts, arps and sequencer and the Atom, but no matter what you do, the MIDI notes ALWAYS have MIDI jitter. Some notes are recorded perfectly (a few of them actually), some ohters have midi jitter, placed here and there in a confusing way.
I have gone through many tests with collaborators, even programmers and technicians (we even changed settings in the BIOS, system drivers..) but we were not able to fix it for good. There is no solution to this problem.
You can see that even in the Gregor's loopback test, which reproduces the same problems, on different computers and setups.
Apprently it does not happen to anyone, as I've heard that Jeff (jemusic) does not have problems with a PCI card and I personally have not tested that one. But even if PCI cards could fix it, it wouldn't be that fair, as not everyone has one or can buy one. The DAW should work flawlessy without the need of additional devices.
I have also searched for more information and there are other producers, musicians and composers who reported this, so yeah, I'm not crazy.
Here are all the articles to support this post:
Admiral Bumblebee article: https://www.admiralbumblebee.com/music/2020/07/11/daw-v-daw-s1-fail.html
Admiral Bumblebee video:
Reddit post #1: https://www.reddit.com/r/StudioOne/comments/hq3qbf/studio_one_live_jitter_includes_article_that_goes/
Reddit post #2: https://www.reddit.com/r/StudioOne/comments/hpjq23/midi_is_still_broken_in_studio_one_5/
There are posts from 2012 and yes, it seems like it has been a long time, but the problem is still there.
Last but not least: personally, this has been a completely disappointment. I loved Studio One in every aspect, but I'll keep working in Ableton until they fix this.
PreSonus, there is evidence: why don't you fix this?