It appears as though the eyes are ellipses with a black Fill.
The mouth appears to be a white stroked path with round end caps.
When you make a Compound Path the separate objects become one object so they can only have one fill and one stroke attribute. What you are seeing is the Stroked Path of the mouth as an open Path with a black Fill.
Prior to making your Compound Path select the mouth and go to Object> Path> Outline Stroke to change your stroked path to a filled shape.This can also be done by selecting your stroked path and going to Object> Expand. Then when you make your Compound Path it will look as expected.
When you make a Compound Path from separate objects that have different Fill or Stroke attributes, the attributes of the object that is at the bottom (the lower position in the layer stack) will be applied to all components of the Compound Path.