Been reporting this for a long time now, but with the unvaulting coming soon I figured it might be worth trying again.
Oberon Prime's back tabard has an excessive cloth collision on the lower legs area.
EDIT 04/04/2020: SOOOOOOOOOOOOO
It turns out it's not the tabard cloth physics that's wrong. After checking the Codex and my Oberon Prime noggle, I have discovered that the whole back tabard piece is MISPOSITIONED.
It's sitting lower and tilted at an angle inside for reasons: