Have you enabled DBuffer decals in your project settings? If you’re using static lighting you may need to do this to improve the appearance of your decals. This article has a description of how to do this: UE4: Solution to Decals Not Rendering/Showing in Indirect Static Lighting Tutorial
Here’s a thread that addresses a similar sounding issue: Decals very transparent under static lighting - Rendering - Epic Developer Community Forums
Otherwise, I’m not sure what might have changed across versions to make your decals look washed out. Some before/after screenshots might help diagnose the issue.