The normals on your mesh are inverted. The effect you're seeing is caused by backface culling in the game engine, where faces with normals pointing in the opposite direction of the camera aren't rendered.
In order to fix this, you can try to recalculate the normals to face outside by using Mesh > Normals > Recalculate Outside or Shift + N. If they are ...
To rule out Unity from the equation there are 2 things to try.
1. try removing the key frame optimisation in the inspector / animation settings.
2. Import your exported FBX back into a new blender scene, do you see the same issue in blander.
If 1 fixes it, it’s the Unity keyframe optimisation settings.
If 2 shows the same issue as you get in Unity then the ...