Transcoding UHD ProResHQ changes black level from 0 to -5mV

I mastered a film UHD DNxHR 12bit RGB in an MXFOP1a wrapper. This file passed QC. One of my deliverables is a UHD ProResHQ MOV which I made in Media Encoder 2019 on a Windows machine. It failed QC because the black levels bumped down to -5mV. It was a small enough difference that I couldn't see on the hardware scope without zooming in. Also the SMPTE bars & gamma appeared identical. But pure black is down at -5mV.

(This is not a full range/video levels issue, -5mV is tiny.)

Oddly, if I make an 1080 HD ProRes in MediaEncoder from this same file, there's no issue, black at 0mV.

To test if the problem was the input codec I made mezzanine files DNxHR 10bit YUV & Cineform but the resulting UHD PR HQ has same -5mV issue.

If I turn off "Render at Maximum Depth" the black levels issue goes away, black is at 0mV, but cannot use this file because the quality is reduced (visible noise/banding)

Upgraded to Media Encoder 2020 & results are same.

Then I tried this on a MacOS Media Encoder & got the same -5mV result in the UHD HQ (very strange)

I then made the UHD ProRes HQ in Assimilate Scratch & the ProResHQ was identical to the source & passed QC, all black levels at 0mV.

Haven't located anyone else reporting this issue but would like to use Media Encoder for ProRes delivery in the future.