When you're inside the command block GUI and you change the size of the window, aka the screen resolution of Minecraft (ex restore down or maximize window), the text in the command block will disappear. All of the buttons in the command block will fade out, too. You have to close the command block and reopen it to see the text again. This has been an issue since the new command blocks we're added.

In the first picture my screen is maximized. Then I restored down the screen and the text disappeared. Then I maximized it again and the text is still gone.