Mushrooms can spawn on the sunlight places when you discovered their landscape at night. This happens in every biome, but mostly in swampland and roofed forest. They also generate on the tops of trees! Normally, mushrooms shouldn't be right under the sky, they must have some cover.

How to reproduce:
1) Explore a world at night.
2) Find some mushrooms on a top of a tree or right under the sky.
3) Remember the coordinates.
4) Create a world with a same seed, tp to these coords (you will find there are no mushrooms).
5) Create a world with a same seed at 1.7.4, set night time & tp to these coords (no mushrooms again).