Same here It was working when 6.3 came out but as of a few days ago two way sync on Last Read no longer works. Only Fire to Kindle is working.
Syncing Bookmarks works both ways.
Additional bug I have found. When clicking on importing bookmarks from Kindle to the Fire the place in book it takes you to loses random formatting. sometimes it's margins that is everything is shifted left and other times it's italics and bold. The only way to get it back is use the GoTo function and enter the location of the bookmark manually.