As a workaround, using 2 .jpg’s. (1 opacity/1 diffuse).
New to this UE world, always Max/Corona Arch Viz but Datasmith convinced to give this a try. Not exactly a one button solution translating Corona to UE.
This might not be very helpful in your case: A save way to get unreal to recognize opacity is with an alpha channel that has the opaque parts masked out white. Then save it as 32 bit. - I remember having problems with png files too.