What seems to be happening, is that when I export, the colour looks like I have applied the lumetri technical preset "Full to Legal Range". When looking at the lumetri scope on a file that I have exported, the waveform shows that the range from black to white has been constricted as if that preset was applied, even though I had not.

I tried applying the "Legal to Full Range" preset to my original timeline before export, and the waveform shows black being below zero. But after export the output file looks like I expect and when I bring it into Premiere and look the scopes, black and white are where they should be.

I tried exporting the same materials in Premiere Pro 2017 which I still have installed, and everything worked perfectly without having to mess around with lumetri technical presets. This issue is a bug in Premiere Pro 2019 (13.1.2) and it breaks the ability to trust in its exports.