Particles using Distance Field collision can have their collision controlled by "r.DistanceFieldAO".
It appears that the condition in "ShouldPrepareGlobalDistanceField(...)" from DistanceFieldAmbientOcclusion.cpp to create the SDFs does not always check if Niagara particle effects need SDFs.
1. Download the Licensee's test project and open (see addl info URL)
2. Open the map "/Game/DFAO"
3. Observe the particles are colliding with the cube "Cube"
4. Execute the console command "r.DistanceFieldAO 0"
5. Observe the particles stop colliding with the cube.
6. Execute the console command "r.DistanceFieldAO 1"
7. Observe the particles are colliding with the cube again.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-368942 in the post.