Thank you for reporting a bug to Unity.
We have reviewed the issue carefully, and in this case, the Z-fighting issue is expected as both the Decals and the surface on which it is projected are opaques are in the exact same position. In order to solve this, adjust the Mesh Bias accordingly (see https://docs.unity3d.com/Packages/com.unity.render-pipelines.universal@12.0/manual/decal-shader.html#mesh-bias-type).
Today we will be closing this case. Thank you again for taking the time to report this issue, and please let us know if there is anything else that changes the impact or severity of this issue.