Hi,
The "Follow Displaced Surface (Corona)" effect is designed to be used in the UV mode (it evaluates the fpItem.distUVW values). If you want to use the XY Surface mode, you can use the "previous" version of such an effect, which reads the "Color Tint" values (fpItem.tintColor.x) from the Material rollout. That should work just fine. Please find attached the modified scene (240402_ForestDisp_iToo.max).
I hope this helps.
Best regards,