Just in case anyone else runs into the same issue — here’s the fix:
As mentioned above, go into the BP (Blueprint), and under Face → AnimClass, assign ABP_Face_PostProcess.
Then, in the viewport, select your Metahuman, go to the Details panel, find Face, set Animation Mode to Use Animation Blueprint, and again assign ABP_Face_PostProcess in AnimClass.
Finally, save your work — done! 
