Version Found: RLDRAM3 v5.0 (Rev. 1)
Version Resolved: See (Xilinx Answer 69037)
Data mask does not currently work for MIG UltraScale RLDRAM3 designs because it is incorrectly tied to GND inside the IP.
In order for data mask to work properly, the following HDL change is required:
Open rld3_default_0_mig.sv and modify line 487:
07/25/2014 - Initial Release