I tried a little indoors experiment with my D700 and D7100 with an SB800. I chose shutter priority, auto ISO and set a shutter speed of 1/500. This gave an aperture of F3.5. In the shooting info display I see TTL over FP. The flash mode was TTL BL FP. I changed this to TTL FP and still I got TTL over FP in the shooting info display.
I then changed the shutter speed to 1/200. The shooting info display no longer showed FP, just TTL. I think therefore that FP will only be shown when the shutter speed goes above 1/250 or 1/320