Adding an ambient occlusion decal material to a decal affects objects with "Receives Decal" disabled.
Tested in: 4.23.1 CL#9631420 4.25 CL#11341939
1) add an object in a scene
2) add a decal to the scene that intersects with the added object
3) set the decal to use a material that is configure to be an ambient occlusion decal material
4) see that the decal is adding occlusion to the intersection of the decal with the object
5) Disable "Receives Decal" on the object
Result: The decal is still adding occlusion to the object
Expected: No occlusion on the object
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-88716 in the post.