Why is the F-35B's elevator not in neutral position when in vertical flight mode?
In this video you can see it better I think: 
                
            

I drew on a screenshot of the video from the side where you can see the air moving under the aircraft.
Compare this to the Fig. 1 in the paper linked by &quot;Anonymous Person&quot; here in the comment section of your question: http://ltces.dem.ist.utl.pt/lxlaser/lxlaser2016/finalworks2016/papers/03.2_3_366paper.pdf
As you can see angling the tailerons down significantly widens the gap between the leading edge of the tailerons and the trailing edge of the wings which allows air to be pulled along with the exhaust gases (drawn in blue). The paper calls this &quot;entrainment&quot;.

Again I drew on another screenshot of the video and illustrated possible air currents that can freely move around the airframe: aft of the exhaust duct and to the left and right of the vertical stabilizers. As you can see the flaps are also extended which helps guide the air around the wings. There probably is a similar airflow around the front section of the fuselage where the wings are not blocking the flow and the fan is pushing the air down.
In a vertical descent or climb the tailerons also create less aerodynamic drag in this configuration than in a leveled attitude so the pitch inducing effects due to the large surfaces at the rear are reduced.
It could also be beneficial to angle the tailerons trailing edge down for a vertical landing or a short takeoff to increase the lift at the rear end by blocking the escape path for air trapped under the rear end of the aircraft. Essentially preventing the cushion of air to escape at least when in ground effect.