Diagonal tearing in vertical orientation

[BMF54123]

When playing in vertical orientation, there is a diagonal tear across the screen where it is showing the current frame on one side and a previous frame on the other (easily noticeable in Mad Planets when the screen flashes; note that the nose of the ship is also skewed):

For whatever reason, this issue does not happen if the screen is rotated horizontally.

[EDIT]
Changing vsync_adjust from 2 to 1 in MiSTer.ini also seems to fix the issue. I don't believe it's a problem with my TV or monitor, though, as the tearing is not visible on the MiSTer menu overlay when scrolling through files.