After a second look I kind of see what you mean. If I replace the Shadow Catcher with a standard physical material the Decal material renders as expected. It seems like the only light it receives is just what is reflected from your cube material (which is next to nothing), and nothing from the hdri, or any other light added that I added.
If I change the cube color to white the Decal renders the correct color although very dark, again because it only seems to be receiving light from the reflection of the cube. If I move the cube closer to the decal it receives more reflected light and gets closer to the correct color.
Long story short...You are correct, I believe this is a bug!