Comme ceci ça marche chez moi :
1ère commande : Repeat, Unconditional, Needs Redstone.
/execute @a ~ ~ ~ detect ~ ~-1 ~ minecraft:air 0 /fill ~2 ~2 ~2 ~-2 ~-2 ~-2 minecraft:air 0 replace minecraft:stained_glass 11
2ème commande : Chain, Unconditional, Always Active.
/execute @a ~ ~ ~ detect ~ ~-1 ~ minecraft:air 0 /setblock ~ ~-1 ~ minecraft:stained_glass 11 replace minecraft:air