I have noticed that when sending from Color to FC, although the render is done to Prores or 10b Unc, the new "From Color.." sequence comes with "Render all YUV in 8b".
Careful because any render will crunch the picture to these 8b.
I think you are pointing in the right direction Walter.
I sent a mix of Prores and XDCAM 422, but the sequence I sent was set to "render in 8b YUV".
The new sequence that FC creates keeps the setting of the original.
Obviously this is not a bug, but something to don't forget to change if rendering again in FC.