I have same problem, tested it on Stable, Fallback and CE builds, all of them have the same issue with big note timings, I'm using laptop and I thought it's happening because of my graphic card, I tried switching from discrete (GeForce 930m) to integrated one (Intel HD something), but it didn't help (however I got a FPS boost from that). I also thought it may be ghosting, but changing keybinds didn't help either, also there wasn't any ghosting (I'm using laptop keyboard).
But when I enabled VSync, getting that fancy animation from properly hit big notes became much easier, however it's impossible to play with VSync for me since it adds some noticeable input lag that makes keeping the accuracy a torture. I tried to use different FPS values and with lower FPS it was much easier to hit big notes properly compared to higher one.
Weird thing is I'm still getting full score from big notes even if the animation of proper dual hit isn't shown, I did the same tests Stynero did, and I could confirm score is given properly. So it may be not a problem for gameplay but seeing the properly hit notes would make the playing experience more pleasant, so I hope it gets fixed.
But when I enabled VSync, getting that fancy animation from properly hit big notes became much easier, however it's impossible to play with VSync for me since it adds some noticeable input lag that makes keeping the accuracy a torture. I tried to use different FPS values and with lower FPS it was much easier to hit big notes properly compared to higher one.
Weird thing is I'm still getting full score from big notes even if the animation of proper dual hit isn't shown, I did the same tests Stynero did, and I could confirm score is given properly. So it may be not a problem for gameplay but seeing the properly hit notes would make the playing experience more pleasant, so I hope it gets fixed.