I tried to play Minecraft without the mouse, selecting everything with Tab and Enter, so I had set my GUI scale to 2 and I wanted to set it to Auto. I went to the Video Settings, I hit Tab enough times to reach the button and I clicked Enter to change it. Although the number DID change, the actual GUI scale DIDN'T change, so I continued to hit Enter but it still didn't change. I hit Tab until I reached the Done button to see if that would work, but it didn't. When I got back into the Video Settings, the number was still wrong.
How to reproduce:
Go to your Video Settings.
Hit Tab several times until you reach the GUI scale button.
Hit Enter to change the GUI scale.
What I expected to happen was:
Both the number and the actual GUI scale would change.
What actually happened was:
The number changed and the actual GUI scale remained unchanged.
Restarting the client after changing the GUI scale using keyboard only, will update it, indicating that it does get properly saved to options.txt, it's just not updated immediately for whatever reason.