Quick answer: Unreal physics asset import skipping the Merge Bones option even when set? Some bone-name patterns confuse the merger - rename bones in DCC before import.
Twist bones bone1_twist, bone2_twist; merge should produce one body each. Result: two bodies per twist.
Rename in DCC
Bone names match merger patterns. bone1, bone1_twist. Merger groups by base name.
Or merge manually
Physics asset editor > select bodies; right-click; Merge. Per-pair work; precise.
Audit per character
Each character has its own quirks. The bone-renaming spreadsheet is part of project docs.
“Auto-merge is heuristic. Heuristics fail in specific cases.”
If your physics asset has extra bodies, the merge heuristic is suspect. Manual merge is the fallback.