Superflat customization no longer accepts version 2 preset strings, silently mangles them attempting to update them

Preset I used: 2;1x7,40x42;1;

What I expected to happen:
40 layers of Iron Blocks, 1 Bedrock layer on the bottom and a Plains Biome
What really happened:
Default superflat settings generated in the world

Steps to reproduce:
1) Create a new superflat world in Singleplayer
2) Enter "2;1x7,40x42;1;" into the custom preset
3) Exit the preset selection
4) Hit the "Customize" button under "Superflat" again

You should see that the preset didn't end up staying.