If you're trying to use a grass block you use //set grass_block and if you're trying to use the plant you can copy it and replace a block to the plant by using //replace stone #copy.
Honestly, I think every regular creative player has already commented on these issues and I'm sure staff are currently working on it behind the scenes it just takes time to fix. 1.13 has caused a lot of functionality issues for servers and they have other game modes to maintain besides creative.
