Hey ! It has been a loong time but i was stuck at the sane problem and found a solution.
To access tile metadata imported from Tiled you need to load or get the tile_set
and use the fonction tile_set.get_meta("tile_meta")
. This return a 2D Array: tile_meta[tile_id][meta_name]
Don't forget to check Tile Metadata : On
in the map import setting !