Just to clarify the procedure more precisely, I used a reference white balance bullb, determined optimal aperture @ 1/125 and adjusted the shutter over and under this reference as shown. The EV steps are fixed and no compensation/adjustments were made to account for reciprocity error in this test. If by the RE you mean the color shifting at +8 and above then I believe that is just from extreme over exposure and the scanner doing it's best "auto exposure". I have not done a "proper" long exposure test with this film but I have certainly taken numerous longer then 8 second exposures with it that clearly show it is unaffected at least to about 30 seconds except the reference artificial light isn't exactly white balanced. Picture below is uncorrected.
I had new 400 done from a wedding I shot and the colors came out pretty neutral. I actually bumped up the saturation a little in a couple of the shots. It's no Kodachrome or VC, but it's not 160NC either.
5x7 Eastman-Kodak kit / B&L 135mm Zeiss Tessar + Compur Deckel
4x5 Graphic View / Schneider 180 5.6 Symmar in a Synchro Compur
RB67 Pro S /50 4.5 / 90 3.8 / 180 4.5 / WLF / prism finder / polaback
Random 35mm stuff
The only thing I don't like about Portra 400 is how orange it is. I've pretty much switched to Pro 400H or Ektar for skintones. 400H is better if you want that light and pastel tone, Ektar is easily the best film if you want natural looking punch to the color.