Hi! It seems like in UE5.4 when retargeting animations, it will wipe out all Additive Settings in every Animation Sequence asset. And this will break your meshes in some way.
As far as I know, you have to go through all the ORIGINAL Animation Sequences associated with that Animation Blueprint and search for all Animations that have Animation Sequence not none, and copy the information to your corresponding Animation.
I think you can see your Animation Blueprint go back to normal after you copy the additive information related to your “idle” pose. But additive information should also be applied for other animations as well 
