Create a new project by adding the default Kevin model, then export to FBX. Under Advanced Settings, choose "Mouth Open as Morph", and export the FBX. (Use whatever other target and options you like.)

Normally, when exporting to FBX, the model will have four character meshes: CC_Base_Body, CC_Base_Eye, CC_Base_Teeth, and CC_Base_Tongue. However, when "Mouth Open as Morph" is enabled, all four of those meshes get combined into a single CC_Base_Body mesh with many sub-meshes.

Merging all of those meshes is a very questionable side-effect. However, the most serious issue here is that the combined mesh will now contain multiple invalid/duplicate blendshapes / shape keys.

For example, notice that the resulting FBX model contains duplicate blend shapes for Mouth_L, Jaw_L, Jaw_R, Jaw_Open, and a few others. This is because several of the original meshes contain identically names blend shapes, and the FBX export process doesn't handle this in a reasonable way.

Notice that in a program like Blender, it's impossible to have shape keys having the same name. If you try, it will force one of the two to be renamed. If you import this FBX into Blender, you will see that it has automatically renamed the duplicate blend shapes. That's obviously a problem, since any animations using those blend shapes will not work.

Similarly, if you open the FBX in Unity, and try to animate one of the blend shapes, Unity will only affect the first of any duplicated blend shapes, ignoring the dupes.

Probably the only viable option here is to stop merging all of these meshes together. The downstream effects of doing that are too great.