When renaming the Parent of a Child Blueprint it invalidates the hierarchy of the Child Blueprint and nests its components under the DefaultSceneRoot.
This bug was reported in 4.19.2 (CL-4033788) and reproduced in 4.18.3 (CL-3832480), 4.20 Preview 3 (CL-4135216), and Main 4.21 (CL-415470)
Result: The child's hierarchy gets invalidated, and all non inherited components move under the default scene root.
Expected: The child hierarchy remains the same.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-61008 in the post.