What happened is that if you set the gamemode to 'Hardcore' and set the world type to flatlands then click done the options to set the 'bonus chest' and 'cheats' on appear available but do not do anything. The only way to reset the textures is to change the gametype is to change to any other gametype and change it back.
Duplicate of MC-10209, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being resolved as duplicate.