Hi @Soaringcylinder
Let’s see…
Unreal Engine might now import normals differently between 5.4 and 5.5. Try tweaking some normal import methods.
While importing the mesh, in the mesh options, check the normal import method, and set it to Import Normals and Tangents rather than Compute Normals.
If your mesh was created in an external modeling program such as Blender, Maya, etc., be sure to check that the smoothing groups are set properly. Unreal Engine will often take smoothing groups from your native 3D software. It might explain why they are interpreted differently in 5.5.