This sounds related to a question I had in this thread. TLDR getHierarchyBoundingVectors will return the bounding vectors of a mesh and all its children in world space which you can covert back to local space as needed. In my experience calling computeWorldMatrix(true) before calculating the bounding vector ensures I get the correct values, getHierarchyBoundingVectors() already does this when its called.
If you are finding different behavior maybe you could try to reproduce it in this playground: https://playground.babylonjs.com/#4F33I3#793