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The Symmetrize Action operator also updates Action Constraints, and even alters its Action to have identical (but mirrored) F-Curves for the bones. This now uses the new API for layered Actions, (hopefully) correctly storing the F-Curves and Action Groups. I suspect there is still a bug in there, but that's now equally there for legacy & layered Actions. Pull Request: https://projects.blender.org/blender/blender/pulls/128039