Last game I played I had two cases where some of my luxuries would go missing. That is, they were in my territory and within city radius and improved but would not show up as available for me or for trade. They used to show up as available but at some point they stopped (and I checked and the hexes were NOT pillaged). This happened at different times during the game (gold in the midgame and spices near the end of the game). I don't have any more specifics on how to recreate this but it was most definately a bug. Probably something to do with the neverending trade cancellation bugs but I just wanted to bring it to attention if any developers monitor these threads.