Had no response on Autodesk forums.
For some reason, Render to Texture is just completely broken for all versions of Max I currently have installed (2016, 2018 & 2019.) It did previously work when 2016 was the only version on my machine.
I can see the "CompleteMap" like result that's in the preview window but every texture outputs as pure black - with the exception of Normal maps coming out with the correct background colour but nothing else.
Are there any solutions or workarounds here?
I'd prefer to avoid external applications so that my workflow remains fast and contained.