RenderTarget2D - SurfaceFormat.Alpha8 gets ignored?

Monogame Version:
OS: Windows 10 x64
Rendering: DX11
Profile: HiDef

Everytime I create a Rendertarget that should use Alpha8 as surface format, it still uses the deault Color format.
Can somebody tell me why? I would like to save as much memory as possible and I actually only need 1bit per pixel. But since there is no 1bpp surface format, alpha8 would be a good solution. 32bpp is just a big waste of memory for my purpose.

RenderTarget2D target = new RenderTarget2D(GraphicsDevice, test.Width, test.Height, false, SurfaceFormat.Alpha8, DepthFormat.None, 0, RenderTargetUsage.PreserveContents);

If I check “target” right after this, it still defines Format as Color and if I use the GetData() Method it requests a 4byte per pixel array.