I don't think it's the UVmapping.  Here's a quick check inside P9.  The left square is the P4 square, and the right one is the P9.  Looks the same.  I also tried the P7 square, still the same.

I use squares for backgrounds all the time, and haven't noticed any problems in P9. 

Makes me wonder if your square prop somehow acquired some bad settings, such as the V Scale on its Image Map node?

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