OpenGL textures always show up black

I recently finished a project that mainly focused on rendering into Unity with OpenGL. Rendering-wise everything works great (primitives, etc.) but all my attempts to render any type of textures inside Unity end up just plain black.

The same exact OpenGL code put in a separate executable works just fine but when moved inside Unity, all textures go black.

I render everything to an FBO and pass FBO's color attachment to Unity to construct a Texture2D from it. Even textures rendered inside the FBO are also black.

I was wondering if anyone else had this issue before or knows how to solve this.