I see, in your photo the camera is not pointing straight down. In my video you can see the gimbal does point straight down 90°.
When the motors are
not running, I can tip the Mini rearward, forcing the gimbal against its mechanical limit which pushes the camera forward. But when I bring the Mini level again, the camera resumes its initial position to again point straight down.
When the motors
are running, if the Mini is tipped rearward and the gimbal comes close to the limit of its mechanical range of motion, the gimbal jumps the camera a few degrees up to avoid touching that limit. The camera then stays at that position and does
not resume its initial straight down position. You can see that at 1:55 in my video.
That will happen in flight if you have the camera pointing straight down while stopped in a hover, then suddenly give full speed reverse stick input. Or you have the camera pointing straight down while flying forward quickly, then suddenly ask the Mini to stop. In both cases the Mini will pitch rearward bringing the gimbal near, or against, the limit of its mechanical range of motion. When that happens the gimbal noticeably jumps forward.
Are you saying your gimbal only ever rotates that far downward, and never all the way down to 90° vertical. Have you tried a gimbal calibration?