That's the way McDonnell Douglas rolls. The opening is sealed by doors, but not fully. From their patent:

[![enter image description here][1]][2]
[![enter image description here][3]][4]

Click either for larger size.

At larger deflection angles as your photo shows, the doors—in this case "the triangularly shaped door (42)"—do not fully close the gap, which is "permissible":

>While it is important that the cutout portion be covered by fairings or doors during the cruise attitude of the stabilizer, it is less important in the extreme positions of takeoff, climb or landing, all of which occur at lesser airspeeds. Thus, while one of the doors is pivoted inwardly on one stabilizer surface to permit stabilizer movement in that direction, the extreme pivotal movement of the stabilizer may cause a gap or air space be tween the other door and movable fairing on the other stabilizer surface. However, **this is permissible** since it occurs only when the aircraft is in a non-cruise attitude.
>
>— Backlund, J., and R. Gibbs. "[Fuselage seal][5]." U.S. Patent No. 3,756,529. 4 Sep. 1973. Current Assignee: McDonnell Douglas Corp


  [1]: https://i.sstatic.net/kxjSdm.png
  [2]: https://i.sstatic.net/kxjSd.png
  [3]: https://i.sstatic.net/aR0oSm.png
  [4]: https://i.sstatic.net/aR0oS.png
  [5]: https://patents.google.com/patent/US3756529A