I found solution for everyone who has similar problems with corrupted animation after adding root bone to mixamo animation fbx file by a blender mixamo plugin. The cause of the problem is too new version of Blender - the plugin does not properly work with the newest one. I have a version of 3.5.1 and that was the problem. After installing Blender 2.80, the plugin does its work and root bones are added perfectly.
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