I am assuming you are seeing something like this?
Decal Actor Applied
This is not a bug, but just how decals function. Think of a decal like a sticker in which you can only apply it to a single flat surface and does not wrap around objects.
Decal Arrow Direction
The arrow connected to the sprite of the decal indicates forward facing direction in which the decal will be applied.
Let me know if this is the issue you are experiencing, or if it is something else. A screenshot of the issue would help me visualize your issue and determine the troubleshooting steps to take.
Thank you,
Andrew