The carpet is not transparent as it was in previous versions. Instead, it loads the transparent pixel as sort of an average color of the rest of the texture, as it would for stone blocks and fence gates.
Please allow for carpets to be transparent so that creative building players and map makers can continue to freely adding content without limitations.
Although under normal circumstances, the carpets load textures from wool textures (Which shouldn't be transparent), with the updates allowing us to change models, we can make the textures different. Meaning that the lack of opacity in carpets is limiting resource packs.