All JSON file datapack shaped recipes failing in 1.13

All shaped recipes fail. There is simply no action, nothing appears in the output window in the crafting screen. At first I thought it was just for 2x2 but it's also 3x3. I have verified all the "working examples" (Piston etc) given in the official Wiki and in the git repo for JSON recipes. I've also used the Destruct7on recipe generator. All shaped recipes fail. All shapeless recipes succeed normally.

Has there been a syntax or JSON structure change? Is there some additional dependency for shaped recipes that I'm missing?