Quick answer: Unreal physics asset using the wrong physics body for a skeletal mesh? The asset's body indices must match the skeleton bone names; rename a bone and you've broken the binding.
Renamed Hips to Pelvis in the skeleton. Physics asset still tagged 'Hips'; collision is on a phantom body.
Re-bind in physics asset editor
Open the physics asset. Right-click each body; Bind to Bone. Pick the renamed bone.
Or use the rename tool
UE5: Physics Asset > Tools > Update Bone References. Walks the asset; flags mismatches.
Avoid bone renames after asset creation
The renamed-bone cascade is broad: physics, anim BPs, control rigs all break. Establish naming conventions before authoring.
“Physics asset bones are string-bound. Strings rot when bones rename.”
Lock bone names in your character pipeline. Add a CI check: bone names match the canonical list; renames require explicit approval.