Thanks a lot.
I finally found the problem with the <ruby> tag at my end. The embedded Japanese font I had was causing the characters inside the ruby tag not to be displayed. I might need to dig further why it happened since it only shows up on "vertical" writing mode or you might be correct that it might be an issue with KindlePreviewer.
I'm using ePub3 code "-epub-writing-mode: vertical-rl;" in the CSS of the BODY tag to display the whole page in "vertical right to left". It works for "Kindle Paperwhite", ADE 2.+, and iBooks 3. I haven't gotten to testing it on the Nook or Kobo if it works on them too.