Lots of commands like /gamemode are not recognized as valid commands in the newer snapshots while you are in survival mode. Please see the attachments below. Sorry if this is a duplicate - I couldn't find any report like this, but I might've missed one! 🙂
EDIT: The bug does not occur if you create a world in Creative Mode, and use commands to go to Survival Mode.
Not all commands are available with cheats disabled.