The way it should be working, if IIRC, is that 0 Level, the enemies should vary from at level to no more than +1 level. So that would mean, conning from white (at level minion) to orange (+1 lvl Lt.).
If you add "Solo Bosses", that might give you a few Red (+1 Boss).
So if you are seeing no white at all, that's a bug, but I would agree that you should still expect to see Yellow/Orange for 0 lvl diff.
If you go to -1, then everything goes down, so it should be mostly a sea of blue & white with the occasional yellow, and rare Orange (if you have set it to solo bosses)