Basically, the glitch is that fill, setblock, and clone commands are all useless. Upon activation, NOTHING happens. The reason this issue is major is because these 3 commands are a major cornerstone of Minecraft mapmaking.

To reproduce:

1. Start up a singleplayer minecraft world.
2. Try to use any of the commands listed above with correct formatting.
3. You should see that the command was executed, however nothing happens to your minecraft world. This second part is the glitch.

Recommended actions to take: Make fixing this bug top priority. Minecraft's popularity is as a result of player made projects such as custom maps.