Hi
Here is an issue with mirroring animation with Mirror Data Table. You can see here:
What problem it could be here? It works fine with native UE5 skeleton. This girl skeleton was created within Mixamo. Could it be because of slightly different (0.003cm) dimensions of the bones?
Did you try adding the mixamo Hips bone to the MirrorDataTable before mirroring? I had a similar issue mirroring a mixamo skeleton but adding the Hips bone to the MirrorDataTable fixed it for me.