I just noticed that CSM shadows from stationary directional lights do not work when Mobile-Multiview is enabled. Neither on Oculus GO or Gear VR with Samsung S6 / S7.
With Multiview off, the CSM shadows work as expected for movable objects.
I have 4.19 with a single stationary directional light in the scene and cannot get CSM shadows to work on the GO. (They do work if i set the light to moveable but not stationary or static).
Did you do anything special to get yours to work usnig a stationary directional light? (also are you sure it worked on Oculus GO?)