Title says it all, the scene color node is only returning a black texture when using the 2D renderer.
Technically it does. The problem is the “scene color” is really just the camera opaque texture, which is a copy of the scene after all opaque objects have been rendered. When doing 2D stuff it’s rare to have anything that’s part of the opaque queue range, so there’s probably nothing in the texture to sample.
Make them opaque. Use an alpha test shader, or just modify their material to use a queue of <2500. However you might have other rendering issues if you do that.