Posting this here to help others, and I thank God for showing me this super simple fix!
If you’re using a Metahuman as a playable character, and you noticed that your Metahuman’s arms (ie. the “Body” mesh) is not respecting LOD settings in the LOD Sync Component, it’s because the “Body” mesh is actually no longer named “Body”! 
This can happen if you copied and pasted in your Metahuman’s modular parts, but you manually set the pre-existing Skeletal Mesh Component inside your Character BP to be the Metahuman Body mesh (such as, “m_tal_nrw_body”):
However, the ACTUAL NAME of this Skeletal Mesh Component which is built-in to the Character Blueprint is CharacterMesh0 and it unfortunately CANNOT be renamed. And since your LOD Sync Component is looking for a mesh named “Body”, and it cannot find a mesh by that name, it does NOT work. Thankfully, this is easy to fix.
Simply do the following:
- Select your LOD Sync Component, and in its Details panel, under “Components to Sync”, find Index 0 and change the “Name” from “Body” to CharacterMesh0
- Next, scroll down to where it says “Custom LODMapping”, and again change the first entry from “Body” to CharacterMesh0
Your Metahuman body mesh’s LODs will now work correctly! 