Still happening in Pre-Release 2.
Yep. It has to be a bug, because in my survival world in Minecraft 1.9 Pre-2 my skeleton grinder with a dungeon spawner does not spawn anything. Hope it'll be fixed before releasing 1.9.
I have the same problem in my Minecraft survival world on 1.9 snapshots (pre-2). I hope it'll be fixed in Minecraft 1.9 release.
Okay, so I explored another Mansion and there was loot in every chest except for the chest room. It probably is an intended feature, but I think if there were at least some gold nuggets, iron ingot or some food (or some other fairly cheap materials) in the chest rooms, it would be much better...