Fixed by commit [1291fe], with the patch contributed by Tomio ((有坂 富雄). The correct command to show the use transform_xy would be something such as:

plot3d (sqrt (abs(sin (theta))), [r, 0, 1], [theta, 0, 2*%pi], [transform_xy, make_transform ([r, theta, z], r*cos(theta), r*sin(theta), z)]);