Hmmmm, let me guess
The combination of ISO 80 + f4 against that moment EV value, need slower shuttle speed to get enough light to obtain a proper exposure. As 1/400 is too fast yet allow only enough lights to reflect from the plant but fail to capture enough lights from the background. Thus the background is underexposure thus it is black.
I ever experience this using flash at a very fast shuttle during day shoot too. Sometime I purposely do it to get rid of the ugly background.
Right ?
