To replicate:

• Start making a new world
• Change the world type to SUPERFLAT
• Click on "Customize"
• Click "Presets"
• Try entering any custom preset (there is an sample below)
• Click "Use Preset"

What should happen:

The list of blocks on the current screen should be changed

What actually happens:

The list is changed back to "Classic Flat"

Sample preset (for you to use):

3;minecraft:bedrock,30*minecraft:concrete;1;

Note: this only works when using a custom preset. Selecting a pre-made one (The Void, Redstone Ready, Water World etc etc etc) will work.