Unable to spawn Wither bosses when a block is placed below of one (or of both) of the structure ''arms''

Wither bosses are unable to spawn if a block is below of one of the ''arms'' the structure has or below both. The direction the Wither spawning structure faces in doesn't matter at all, the Wither boss wont spawn. (tested)

http://gyazo.com/c564bf8d73729e804d789df6c13d88dd

It worked on 1.7+ and prior releases.