Operating system: Linux Mint 19
Graphics card: GT 630
2.80 (sub 39), branch: blender2.7, commit date: 2018-12-25 16:36, hash: 60b930af3e73
I hesitated two days now to report it since usually i'm just embarrassingly stupid and there is no bug at all, but i feel helpless. Grab the Root-IK (big arrow) in pose mode and lift it up. Note how the mesh is moving much faster than the bone, resulting in deformations.