my terrain details are rendering black when not having a back geometry in VR

Hello, I've a problem and I'm begging for help. I'm actually working on a VR project using Oculus Quest 2. I've some "detail mesh" and "grass texture" in my terrain.

When I'm working on it or in playmode they look fine, but when I do the build and launch de app on the Oculus, those details are doing something strange. They renderice well when they've a background, when they have a geometry behind, but when they hit the horizont, when the background is the skybox, they look black, like the alpha is the one that renderices. Here a example:

The grass on the picture it's direclty a grass texture so it has not material, it's just the sprite, but I've the same problem with the others (detail meshes), sooo I don't think it's something about the material.

Thank you for reading me and I hope we can find a way to solve this!