Mushroom with a light level higher than 12 at two or fewer blocks of the light source break instantly when placing or removing a block adjacent to the mushroom.

For example: at the screenshot, if we break the glowstone, any mushroom breaks at the right, but all mushrooms break at the left.

If we place a block on the top of the red nether bricks, at the right only the two adjacent mushrooms break but at the left all of them.