The force of an explosion on a non-living entity is not affected by the power of the explosion, the distance from the explosion, or any blocks that may be shielding the entity from the blast.

To demonstrate the bug, activate a TNT block on one side of a bedrock wall, then quickly activate another TNT on the other side of the wall. When the first TNT explodes, the second TNT will be pushed. Prior to 1.9, the second TNT would not move at all.

This Bukkit patch demonstrates a possible fix:
https://github.com/OvercastNetwork/SportBukkit/blob/9de4c6e27928a02f534fae75b585c0b4c9d08824/CraftBukkit/0142-Fix-explosion-distance-and-occlusion-being-ignored-f.patch