I GMd lockpicking not long ago, going through so. so. so. so many dungeon chest. My net count of Platinum from those is a big zero. It's my understanding there was supposed to be a chance for them, so I assume there's a small bug connected to them at the moment.
Not yet added that I'm aware of. There are always plat on Level 6 tmaps, and chances on the lower levels.
I believe 100% chance of 3 platinum in a L6 tmap chest, and 20% chance of 1 platinum in a L5 tmap chest. Chris has a post somewhere that is kind of confusing about these things though, it says something like "Platinum in L4 chests, patch 46+" but it means "this will happen in some patch after patch 46", rather than how most of us read it as saying "this now happens since patch 46".