/mavros/local_position/odom data wrong

[Kasho-2001]

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Is the odometry in gazebo hallucinating?! We use the iris_vision drone for a university project, where we give out some velocity commands to "/mavros/setpoint_velocity/cmd_vel" In the first 2 minutes the drone behaves exactly as we want, however after this time, the drone suddenly begins to drift in the z direction. After printing out the z-value of /mavros/local_position/odom we noticed, that it has a huge difference to what we can see in the gazebo simulation.
The problem just occurs if we set a horizontal and yaw velocity together. It is so strange, because it works perfectly in the first minutes.

I would appreciate any help!

MAVROS version and platform.

Mavros: ?1.16.0? ROS: ?Noetic? Ubuntu: ?20.04?