It's because they are drawing it to the left of where the API is asking it to be drawn.
We actually try to handle that, since some context menu handlers do the same thing while others draw it where they are told. We ask things to draw in the right half of a space that is twice as wide as it needs to be... But because in this case it drawing in the left half of that space, but also drawing an icon that is wider than icons are supposed to be, parts of its icon are drawing into the right-hand side of the space, which makes Opus assume the handler has drawn in the correct place.
(I'm not sure why they are custom-drawing the icon at all, really, since Windows has had a better way to add icons to context menus for over 10 years now, by just providing an icon handle and allowing the host program to render it as needed. But if they want to custom-draw it, they just need to respect the X coordinates that are passed to the API and/or ensure the icon is not wider than the system's icon metrics allow.)