Disabling the Decal DBuffer in the Project Settings>Rendering causes the Decals to not render on geometry that overlap their respective Decal Actor.
Tested in 4.21.2 (CL - 4753647), 4.22 (CL - 5275346), 4.23 (CL - 4980862)
Expected: The Decal Material should appear on the floor within the volume
Result: Nothing additional is rendered on the floor
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-71258 in the post.