Dark Render of custom Models on Iris on certain shaders

Shader that is being used is the SEUS PTGI HRR 2.1, resourcepack used is the Optimum Realism R2.5.0 WIP. The 'custom grass' appears dark on Iris, (refer to image 2) and the in-game grass appears fine as it should be. Both, the 'custom grass' and in-game grass appear uniform on Optifine with SEUS PTGI HRR 2.1 The 'custom grass' has been added by adding 'multiparts' in the blockstates.