Bug animator with nested character handles root motion from child causing it to offset the child

I set up characters by parenting the character to the main controller - the root,
then move the child character's animator component to the root.

if I try to use animations with root motion on the timeline, the child character's local position will offset based on the root motion's delta position. But the real root is moving normally.

This character structure works perfectly on 2018.3.x versions but broken from 2019.x up to the latest version 2021.1.2f1.

I also already reported the bug months prior, but the support emailed me that this is a duplicate bug and apparently already marked it as "solved" on the issue tracker.
Unity Issue Tracker - Nested Avatar teleports when moving it in the Timeline (unity3d.com)
Unity Issue Tracker - GameObject position changes during transitions in Timeline preview (unity3d.com)

update:
I used position constraint to avoid the child character from offsetting it's local position; while i found a workaround to keep the child's local position to zero, i still think it's a bug that needs to be fixed.