Red mushroom growth behaviour has apparently changed.
Before they could grow inside a 5x5 square minus the four corner blocks

◼︎◻︎◻︎◻︎◼︎
◻︎◻︎◻︎◻︎◻︎
◻︎◻︎◻︎◻︎◻︎
◻︎◻︎◻︎◻︎◻︎
◼︎◻︎◻︎◻︎◼︎

◼︎ = occupied block
◻︎ = air block

It seems that the presence of any block in the four corners now in 17w50a inhibits the growth of the mushroom, unlike previous versions

They grow if the only solid blocks in the corners are under the box of the mushroom cap.

The mushroom can also occasionally ignore the height limiting blocks put above the vertical the stem, growing up to touching it (previously they only could grow to one block below).

Not sure if intended

Needless to say, for farming purpose this is quite an annoyance..