Prior to version 4R4 the floor was opaque when viewed from below, so that you couldn’t see your model. What you are seeing is not a bug but was introduced so that the model became visible from below the floor.
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To render without the screen dots, use AA Half mode. So double the size of your document and then press AA Half.
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For more control, turn off the floor and instead append a plane. Position and scale as you wish and then use the Tool>Display Properties>BPR Visibility slider to set the degree of transparency.
It’s not possible to render a scene from below so that there is a floor shadow but the floor or plane is completely transparent. You would need to use the plane method above to output separate Shadow passes and then composite in Photoshop.