We have heard this feedback a few times and have verified that we are commanding the gimbal to 90 degrees down and zero degrees in pan and roll. You can see that these are the numbers if you look through your log file.
The issue here is the presence of a head or tail wind which makes the aircraft rotate a bit to handle it. It seems DJI doesn't account for that in the pointing of the gimbal although they probably should.
The way to avoid this is to fly during periods of minimal (or no) wind. We will look into correcting for this in a future release. We want it to be perpendicular too.