When loading a 13w36b world in 13w37a or b, four chunks I had built on converted to one of the new biomes, deleting everything there as if it was newly generated terrain.

Vanilla world with Command blocks used for teleportation only and in different chunks than the ones affected. I transfered an old 1000 by 1000 blocks area to a new 13w36b world using MCedit and set the biome to Extreme hills as it used to be, but as the other chunks weren't affected, that can't be the problem.

The only blocks existing in those chunks that are nowhere else in the world are lit redstone lamps on item frames, but even when I made sure to take out both the lamp and the item frame before updating the bug still occurred.

Does not crash the game, but deleting weeks of work doesn't seem intended.
I took before and after screenshots with seed and coordinates, but I "don't have permission to create attachments for this project", so I uploaded them somewhere else and made direct links.
Before: http://imageshack.us/a/img15/4677/uw8z.png
After: http://imageshack.us/a/img196/1867/h4be.png