@daniele brought up during the last meeting that the gamut mapper compresses the area of the spectral locus such that there is a ‘slice’ by the P3 red primary which is empty. This is also the case, to a lesser extent, with the P3 blue primary.
But thinking further about it, I believe this is inevitable with any gamut compressor. Since the red to green edge of the AP1 gamut lies hard up against the edge of the spectral locus, if the compressor is to bring colours outside that edge to within the gamut, it is inevitable that colours on the edge need to be ‘pulled in to make space’.
So a value can only end up at the P3 red primary if it starts outside the spectral locus.
We should also remember that if the gamut compressor is applied just after the IDT, then subsequent grading is still able to push a colour back to the P3 red primary if that is creatively desirable.