I am submitting this same issue. If I have clicked on an artboard, the ruler units will update to reference THAT artboard, but if even a sliver of the artboard to the left is visible in my Illustrator window and I select the vertical ruler to slide a guide over (to the right), the ruler updates to reference the artboard to the left. The only workaround seems to be ALWAYS repositioning the artboard I am working with to be the first visible artboard next to the vertical ruler.
I use the latest version of Illustrator per the date of this post.
I am submitting this same issue. If I have clicked on an artboard, the ruler units will update to reference THAT artboard, but if even a sliver of the artboard to the left is visible in my Illustrator window and I select the vertical ruler to slide a guide over (to the right), the ruler updates to reference the artboard to the left. The only workaround seems to be ALWAYS repositioning the artboard I am working with to be the first visible artboard next to the vertical ruler.
I use the latest version of Illustrator per the date of this post.