OK I re-read your post and followed each step. I have duplicated what you are talking about.
I think this is a case of the focus assist (in effect a digital zoom) showing only a very small subset of the entire frame, without much light to work with; the EVF has no more ability to 'gain up.' At 45mm, the kit lens is very slow, f/5.6.
To confirm my suspicions, I put on a 45/2.8 adapted lens and repeated the test; with the adapted lens at f/5.6, the focus assist image was exactly the same level of brightness (darkness) as the kit lens.
I turned the aperture to f/2.8, and it brightened right up (2 stops is 4x brighter of course), easy to focus even at maximum focus assist magnification.
Do the same test, turn on maximum focus assist magnification, and zoom out to 14mm, the lens opens up to f/3.5 and is quite a bit brighter.