What codec are you exporting to? It could be that the codec changes the color values somewhat, as a codec is all about keeping vital information and throwing away what is not, a process called compression.
Changing the sequence settings and export settings and also enabling "display color management" in general preferences hasn't done anything. I thought I might have solved it when swapping to Rec 2100 HLG since the preview window for the export finally matched the source window, but the video after export still came out different in colour. Also tried applying the LUT fix that seems to be semi-official for this issue.
Given up now and taken the simpler solution of just matching the website background to the video instead of vice versa. No idea why this is happening but hopefully Adobe fixes it.
I am assuming this is something Adobe will never be able to fix, as it does not exert that level of control of what codecs do to the moving images.
Unlike their own propietary PDF file for printed material (i.e. certified PDFs), color accuracy all the way throughout a workflow upto hexa decimal values is very, very hard to attain. Not only the encoder is in play here, but also the decoder used to showcase the video onto the screen.
I think you made the right choice in approaching this the other way around.