Not a bug, that's just how it works now.
1) Open Security app and then "Data usage".
2) Click the "Restrict data usage" button in the middle of the screen. (If that button is missing, continue to 3)
3) Click the "Set data usage limit" button in the middle of the screen.
4) Click "Data type plan" and select "Monthly data plan" (even if it's already selected).
"Monthly data plan" category will appear on the screen with settings like "Peak data limit", "Set peak data usage" and "Custom data plan".
5) Set the "Peak data limit".
6) Go to previous screen (main Data usage screen), and click the Settings icon on top-right corner.
You will now see the previously missing "Data plan", "Set data usage" and "Data usage warning" settings.
Inconvenient, but that's how Xiaomi programmed it.