That's two things happening at once: 1) Those areas are below visible black in P3 color space and 2) What IS there is driving the Vimeo compressor crazy.
One of the biggest differences in P3 is in the gamma curve, especially in the low-mids to lows. For an example, let's say on a scale of 0=black and 1023=white, your computer monitor in sRGB, or Rec.709 can easily differentiate between values as low as 25-50; that is, on a calibrated monitor you can easily tell that a value is 50 is brighter than a 25 (and that 25 is brighter than 0, as well). Well, in P3, it's basically like any value below about 300 is all pure black. This has the effect of increasing saturation, but it also creates a large safe-zone for "black." The negative consequence of this, however, is that things which were assumed to all be in the black safe zone may in fact appear when viewed in other color spaces. A variation of this is why the garbage mattes in Star Wars, which were invisible in the theater, showed up to the horror of everyone involved when Star Wars was first being transferred to video. Different luminance/gamma curves - and that also means different saturation/values for colors. There is one immutable fact to remember: YOU CANNOT DISPLAY P3 COLOR SPACE ON ANY MONITOR (other than the $40k Dolby PRM-4200).
Lots of times, we simply grade in Rec.709, because the entire 709 color space fits inside the larger P3 color space. So we know that our Rec.709 footage will look the same in P3 once we apply a correcting gamma curve. But Rec.709 is missing many color hues which are crucial to replicating film, so you don't want to throw those out, just for Rec.709/sRGB compatibility. I try to tweak the videos a touch to more closely approximate on your monitor what the shots actually look like, but it's only marginally effective. These shots actually look much sexier "for real" than I'm able to show you. But when the entire thing is locked, I'm going to do a special Rec.709 grade conversion to be able to show more accurate stills.
Great eye; thanks for the comment; keep them coming. I don't want to miss a speck, and I know you guys won't let me! Also, if you want more detailed information and videos on the specific shots and processes, they're on the starwarslegacy.com forums. There's just no way to put all that discussion into this one thread here, so my idea is to post overall updates here, and then if people want to dive in further and get more details, that's what the forum is for!
_Mike