I don't trust the monoscope pattern in the Dreamcast 1.29 release. The Genesis and SNES ones looked fine, but it's definitely not a perfect square (the red) on my monitors when I leave the settings as they are.
2.43% wider than it is tall. Tried this on multiple monitors.
I just double checked it with the Genesis version. I get a perfect red square. So yeah, check the DC version against the other ones and let me know what you find.