The red mushroom blocks don't work properly. Upon spawning and on growing them from a mushroom, they are fine, however, if you break them (with silk touch) then they turn the wrong colour. As items and blocks, they have the brown mushroom texture. They are called red mushroom blocks, they appear red on a map, and they are the correct texture in the creative inventory. However, upon being broken in survival mode, they drop as the brown mushroom texture. They do not stack with the brown mushroom blocks, which they share the texture with, and they do not stack with the red mushroom blocks from the creative inventory. When being placed and broken again, they are still called red mushroom blocks. This bug means that you can't build anything out of them in survival without complex piston mechanisms. Hope this gets fixed soon!