End portal (119) disappears if placed in the End, even after killing the Ender Dragon

According to source code obtained by decompiling Minecraft with MCP, End portal blocks (ID 119) are meant to disappear if placed in the End before defeating the Ender Dragon, and presumably stay in the End if placed after the dragon has died. Instead, when placing 119 in the End, it despawns immediately regardless of whether the boss has been defeated.