To get it short, there is the link to the reddit post I made 2 days ago, so I don't have to write it all over again: https://www.reddit.com/r/Minecraft/comments/fggcvj/custom_grass_block_model_problems/

The topic pretty much explains itself, I tried to add more grass 2D plane textures on top of the grass block model, to create a "lower" version of grass. And yet, I incountered some problems with cullfacing. In the reddit post you'll find a more accurate infortmation of what I mean.
Thank you in advance for all the support you can give me on that.