I happened to encounter the same situation as you did. Here is how I solved in UE4 4.20:
1> Open the Boy01_Mouth_MAT1 in Material Editor
2> Click the Result Node , change “Blend Mode” to “Masked” ,as shown in the attached picture.
3> Hook Alpha of Texture onto “Opacity Mask”
Then apply all the same steps above to other Material except Body_MAT
Good Luck.