Ender chest broken with silk touch drops chest AND obsidian.

Upon breaking an ender chest with my enchanted pickaxe (silk touch, unbreaking II, efficiency II), the chest drops an ender chest (expected from silk touch) PLUS eight obsidian (expected only if breaking without a silk touch). Repeatable, accumulating obsidian each time.

My main screen shows version 1.2.6, but I don't know how to see what sub-version I am using (.55 or .60 are the selectable options as I write this).