Yes, the Y-axis is inverted. Do you see this? I’ve attached an example of the same image saved from Photoshop and Mac Preview (Export -> Windows BMP).
If you open them in a HEX editor, you’ll plainly see that all of the image bytes are written in opposite orders. In the Photoshop file, the bytes are written from the bottom up, and in the Mac version they’re top-down. I suppose the Mac version makes more logical sense, but it’s not the standard.
If you look at the file headers at offset 16, you’ll see the vertical resolution of the image represented as a 32-bit signed integer. The photoshop file is 64 (40 00 00 00). The Mac file is -64 (C0 FF FF FF).