The bug
On the superflat customization screen, the layer icons and the preset icons aren't displayed if no world has been played in since the game has been launched.
To reproduce
Close Minecraft, if it is open
Launch Minecraft
Enter the "Create new World" screen
Select "Superflat" as world type
Click "Customize"
→ ❌ The layer icons don't displayClick "Presets"
→ ❌ The preset icons don't display
Can confirm. Only happens if you haven't opened a world or played on a server before accessing that screen.