[MC-30167] Choosing a different biome does not work on superflat customisation if you typed it yourself

Steps to reproduce

Open up the world creation menu
Create a superflat world, in the presets customizing section, type biome_ and a number other than 0
Create the world, the biome will not be there

Linked issues

You're doing it wrong.
The biome is the number before the structure string, in that example it's the 3:

2;7,230x1,5x3,2;3;stronghold,biome_1,decoration,dungeon,mineshaft

oh! thank you lots, thaats very helpful