Just a thought, but an idea which I will struggle to put into words.
If you were working with a couple of images which were then combined, presumably using layers or something similar, were those images fully 'fused together' before uploading?
I'm wondering if we are just seeing part of your editing reproduced on FB.
Does that make any sense.
I sometimes get similar problems when copying or pasting images between different editing windows if the layers haven't been fully merged.
For instance, one case where this often happens to me is when placing two images onto a pre made 'template image' for printing. I often place two A5 images onto an A4 sheet and print; then cut them apart after printing.
And, when I forget to merge everything first, I sometimes get similar results to what you are seeing.
Changing from an editing format, like Tiff or Psd etc, to a standard Jpeg should automatically fuse any layers. But is something not working correctly here? I'm assuming you are uploading with Jpeg and not Tiff etc.
Like I said, just a thought, otherwise I'm equally baffled.