i think its because of the normals facing. on planes, in roblox studio, we cant see the texture on one side of the mesh, but roblox recently fixed it. you only have to check doublesided in the mesh properties
The reason this was happening is because the cave “darkness” had a “fog” effect, with multiple parts with different transparent objects, really simple
For some reason that could be seen from outside of the waterfall
I left it to only 1 black part and it fixed, no clue.