RVMAT "Material - Basic glass reflectance" does not test Z buffer. Mesh rendered partially in terrain is visible and has a strobe effect on lighting.
renderFlags[] setting in RVMAT file has no option for Enable/Disable Z Buffer test, only Z Buffer write ("NoZWrite").
RVMAT Basic Glass setting:
[[ https://community.bistudio.com/wiki/Material_-_Basic_glass_reflectance | Simple Glass With Reflectance]]