Unable to plant seeds on Minecraft server.

On the server snap.viper.works, I am unable to plant seeds on farmland. They just pop right out of the ground! It worked fine for a bit, I planted some seeds, but then the rest just began popping up out of the ground when I planted them. They seemed to pop out of the ground whenever there was a block update or a random tick.

In the screenshots below, you see that I planted the seed fine, but it popped out of the ground when I tried to plant a seed next to it.