This is expected. Unreals base unit is centimeters, which means 0-1 (the maximum range that can be displayed on your monitor) is only 1 centimeter. It’s not meant to be human readable. Almost the entirety of the scene is outside the visible range, but assuming you are using a 16+ bit render target the data is still there.
Textures can be sampled from Blueprint using the node (I think it’s called) ReadRenderTargetPixel. It is insanely slow, and not suitable for realtime.
Niagara has a texture sampling module, you can open it up and look at it. Sampling textures can only be done in GPU emitters.