“One other thing that may be helpful to know is that, at least on every VFX for film i've been working on the past 10 years, you NEVER apply the LUT to your sources. Instead you apply it as a display LUT, so your sources remain untouched.”
This isn't a display LUT. It is a scene referred linear transform to REC.709 based primaries. Inevitably such a conversion will likely yield out of gamut values via negative RGB. In compositing, that will result in a loss of gamut information, but it is the only method to work in a smaller gamut.
With that said, this should have nothing to do with those out of gamut values in this particular case. The detail in low quantisation float values are being lost, which is typical if handled poorly in float to GPU flip flops, or simply mishandling float data. If OCIO were inadvertently being set to GPU, it would be the pinch point.