I posted this on my thread - but will post again here as I think we’re dealing with the same issue. This has solved some issues with mine:
Key points: Dest SkMesh must have root bone retargeting option set to Animation Scaled && both IKRigs must have a chain that includes the pelvis (and those chains mapped). I found the default UE4 Mannequin IKrig does not have pelvis set up by default - so I just made a 1 bone chain and it worked. Pelvis and Root are locked.
*Update - further testing & @Faulkin’s comments lead me to believe this is not an ideal solution for all cases. I’m not sure to put that on this method or on growing pains in the the UE5 retarget system.