Still a displacement bound issue where you need to increase even more I bet.
Displacement Bounds is in scene Camera units when rendered (which is why it is never a good idea to scale cameras

). As was mentioned above you can enable verbosity to a high level and after the render completes you will see the maximum displace distance in scene Camera units.
Glancing angles on surfaces that are displaced cause lots of grief for MicroPolygon render engines. You can try the pre-dice render property (adding it manually from the Render Properties to your object) to see if that helps if you want to use MicroPolygon rendering for primary samples.
If RT does it with no increase in render times, then that's good and go with that.