This isn't something the app is telling it to do. We set the gimbal one single time to the prescribed setting and the gimbal does the rest.
This could be a gimbal calibration issue. Try flying with the gimbal at -85 degrees instead to see if that fixes it. The mapping outputs would be the same or better anyway.
It shouldn't be anything with the app commanding the gimbal position but Issues Using Older iOS Devices
Try a newer device if you can after you do a full aircraft and gimbal calibration and try using a slightly less than 90 degree setting.