Whenever I try using Alt+2+1 to get the section char (§) in Minecraft, I get ☻.
I also am unable to paste the section char.
I use a Corsair K55 RGB Keyboard and am running Windows 10.\
If you need more information, let me know.
Do you have an 'alt gr' key on your keyboard? If yes, try to use this instead of 'alt'.
Else it may be that your keyboard-layout is wrong set.
Try to change the layout.
Minecraft specifically does not allow you to use the section character at all in-game. The ampersand symbol &
is commonly used as a replacement, but must be explicitly supported where you use it.