Horizon tilt is a real problem that has been reported literally countless times across several FW versions. There are two separate issues that I have been able to ascertain from reports.
1. Tilted horizon. This presents as an horizon that is constantly tilted in a similar direction and angle. This can be a mechanical issue or not. Each case is different. If not mechanical, it can be helped sometimes with a level IMU calibration followed by a standard gimbal calibration or a mid air gimbal calibration. It can sometimes also be helped with a gimbal roll adjustment. In prior FW versions, the gimbal roll adjustment value was volatile, meaning it would not remain set if you restarted your Phantom. The latest FW versions have changed this setting to be non-volatile, so once you set it the value will remain set.
2. Drifting horizon. This presents as a horizon that varies as you yaw. It will be level in one direction, slowly tilt one way, return to level, slowly tilt the other direction, then level again. It may also show as perfectly level to begin a flight, only to slowly drift out of level throughout the flight. Doing a level IMU and gimbal calibration may often give false hope as the horizon may appear to be level for a while, and then suddenly begin to drift out of level. Some people have been helped with this issue with each FW update, but many have not. From what I have read, this issue has been known to occur on the Inspire as well.