Im getting a strange issue with importing .FBX into unity. It does not look like it's an issue with normals. I have noticed on unity the material is classed as 'transparent' and changing this to 'opaque' does fix the issue but i would like to find out whats going on here.
Unity: 2018.3.3f1
Blender: 2.80.0
I've just run into a similar issue. I've not figured out why it's happening yet, but there seems to be a bug report out. Will update if I find out what's causing it.
Haven't found the exact underlying cause but I've had a few minutes to test out a few things. It seems like blender is using the base colour's R channel as both the Red colour and an inverse Alpha channel value if the R value is anything other than 1.0 or 0, ignoring the Alpha channel completely, and keeping G and B channels as the values they should be.
All of the below assume a basic model (a cube, say), and no modifications to anything other than the Base Color property of the singular material applied to that model.
RGBA(1,1,1,1) in blender, Opaque RGBA(1,1,1,1) in UnityRGBA(1,1,1,0) in blender, Opaque RGBA(1,1,1,1) in UnityRGBA(0.9,1,1,1) in blender, Transparent RGBA(0.9,1,1,0.1) in UnityRGBA(0.1,1,1,1) in blender, Transparent RGBA(0.1,1,1,0.9) in UnityRGBA(0.1,0,0.1,1) in blender, Transparent RGBA(0.1,0,0.1,0.9) in UnityRGBA(0,1,1,1) in blender, Opaque RGBA(0,1,1,1) in UnitySo, to reiterate:
1 - RHope this helps!