The skeleton tree search does not search for constraints based off of their full displayed name in the tree. Only their second bone name is used when filtering (despite portions of their full name still highlighting when matched).
This seems to be the result of the displayed widget text (FSkeletonTreePhysicsConstraintItem::GenerateWidgetForNameColumn) not matching the name used for searching(FSkeletonTreePhysicsConstraintItem::GetRowItemName)
In the skeleton tree search box type in 'constraint', or the name of the first constrained bone of a constraint.
Expected: All items with the search text in their displayed name will be filtered for.
Result: Constraint items are only filtered for using their second constrained bone name.
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-84437 in the post.