Visit the tile experts at www.johnbridge.com
I think you will find that the experts recommend two layers of plywood under 12x12 tiles.
The problem could be cracked tiles, which is more serious than an occasional loose tile.
Also, attic floors often have minimum size joists on long spans. That is another condition that is not good for porcelain tile. They will also advise you on spans and joists.
You should keep a good stock of matching tiles to replace cracked ones.