The device I am using is a Samsung Galaxy J5.

Setup: Spawn a wither sideways on a block of bedrock with a 3x3x2 bedrock 2 blocks above the bedrock as shown: (For clarity, soul sand is replaced with glass.)

What I expected to happen was:
The wither would spawn on the bedrock and be trapped as would any wither cage it is in.

What actually happened was:
The wither spawned beside the bedrock, and it would always spawn in that position if spawned repeatedly using the method shown. If the spot was blocked by any block, the wither would spawn in a different position. When enough blocks are placed, the wither would spawn inside one of the blocks.

I suspect this might be intended behaviour as I think it will avoid trapping the wither, but I am reporting this as a bug because it is not consistent with the behaviour in the PC version, and I quote from the wiki trivia, "The wither spawn structure can be built any way, such as downward or horizontal, however the wither will always spawn at the foot of the structure."
However, I understand that inconsistencies between versions might not be considered a bug, in which case I apologize for the inconvenience. Nevertheless, I still want to know if this behaviour is an intended feature.