i can also confirm on the Mixer front. routing to Mixer paths work fine. too bad about the routing from OnFilterRead. the audio team at Unity is literally two people, Wade and Jan, in Copenhagen though this might have changed possibly. i know one or two folks not at Unity that might be able to shed some light on this as well, as they both designed their own audio engines of sorts.
this thread seems to relate and both posters apparently found a solution for it, though it's not explained how:
and here's the original that i think the other was a duplicate of. it was closed as being designed that way, thus not necessary to fix: https://issuetracker.unity3d.com/issues/stereo-pan-on-audiosource-has-no-effect-on-audio-generated-with-onaudiofilterread
so that seems to indicate that 2D panning in Unity is not possible using a custom OnAudioFilterRead - yippee...
but with the 3D position sound settings - have you or can you test that a sound in 3D space is placed to the left or right of a listener? because if not that means that a pretty crucial part of the 3D sound settings is NOT working. volume and attenuation over distance are definitely not the whole picture for 3D audio. you're gonna need azimuth at least. and since both of the Unity posts referenced panning, it was likely assumed this was 2D related and did not cover 3D position.