glTF defined materials within the 3d model (not the one defined in Plume), when their alphaMode property is set to BLEND, are displaying a shadow in the transparent areas, when looked from an angle that is not perpendicular to the surface. Of course, depending how it blends with the background.

Is there a way to make the transparent areas totally transparent?

First image: Perpendicular invisible.

Second image: Changing the angle.

Basically, it is about make the transparent areas fully transparent when the background is darker, or not white.