Which prop? There is none in Poser primitives called solid square.

It should work right with the one-sided square.

And when you rotated your square, on which axis? It should have been the X axis. If you did the Y axis, then you didn't turn it upside down. You spun it around, keeping the top and bottom in the same orientation. I'm just guessing here. I have never seen a texture render the same way when you flip a prop upside down.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)