Blendshape smoothing not being imported correctly from FBX (Blender)

Environments: Blender: 2.93 Unity version: Unity-2019.4.29 OS: Windows 10

Description: I have made a model in Blender containing blendshapes (specifically ARKit iPhone blendshapes). I export it from Blender as an FBX. I then import the FBX into Unity as I use Unity to convert it to a VTuber format using UniVRM. I am aware of the issue with blendshape normals in Unity so when I import it in Unity, I make sure to check the box "Legacy Blend Shape Normals". However, there seems to be another issue related to smoothing. When I view the FBX blendshapes in the Scene view, they look different to how they look in Blender, the blendshapes are broken and not smooth. (see screenshots) I think the issue is to do with how Blender is storing the smoothness information in the FBX? Honestly I'm not sure.

The following screenshots show the combination of jawOpen and mouthSmileLeft and mouthSmileRight blendshapes. It happens with other blendshapes too, such as mouthFrownLeft & right, and the eyelashes are also affected.

Blender:

FBX:

I am not comfortable with sharing the original file as it is for a paid commission.