Hello GD09Scott,
What you are reporting is actually not a bug, and is working as intended. In order to get your decals to display properly you will need to edit a few settings. Firstly, the reason it is not showing on that particular face is because it is not receiving direct light as a ‘Translucent’ decal. By translucent I mean the ‘Decal Blend Mode’ is set to ‘Translucent.’ Use the directions and images to get your decals to render correctly.
Project Settings
Go into your 'Project Settings’ and within the ‘Rendering’ section scroll down to the ‘Lighting’ and ‘Optimizations’ options and check ‘Dbuffer Decals’, ‘Movables in early Z-pass’, and ‘Early Z-pass’ to ‘Opaque and Masked Meshes.’
Decal Material Properties
After making these changes, close and restart the editor. Rebuild your scene and you should now be able to see the decal on the face of the BSP.
Let me know if you are still having difficulties, or if you have further questions.
Cheers,
Andrew