I believe I've figured this out... it's all down to the codec. I feel pretty stupid for not realising that earlier... I converted all the 2100 HEVC footage to Pro Res, retaining the HLG profile. I managed to get 2100 to display identically through an AE comp on my PR timeline, as it does natively on the timeline (AE settings were CM to 2100, preserve profile on footage and 32 bit float, PR color space 2100). The scopes were identical. Then I tried with 709 (preserve RGB on clip. project settings: 709/2.4, 32bpc (probably unnecessary), and 709 space on PR timeline). Visually they look identical, but I noticed a slight shift on the scopes, but it'll have to do for now. I hope that helps someone!
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