While investigating an issue with Cull Distance Volumes, I noticed that AVolume::EncompassesPoint has a broken default implementation when WITH_PHYSX is not defined. If only chaos is enabled it will fallback to using the brush's bounds instead of exact geometry, which will be very inaccurate for weirdly shaped volumes. I believe the GetSquaredDistanceToCollision function it calls works fine in Chaos and this is just a remnant of when PhysX was the only option. There also may be other places in Engine that need to be checked for out of date WITH_PHYSX calls
This would cause very different behavior between PhysX and Chaos for any systems like Post Process Volumes or Cull Distance Volumes that need to see if a point is inside a volume
Effects depend on the exact shape of the volume, comments have linked bug referencing behavior
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-101956 in the post.