ultrasound_freq is not set in data/config.ini if its 8

[itssme]

I am experiencing a problem while flying with two AR drones. Sometimes the ultrasound sensor is sending incorrect values. I am controlling each drone with one separate PC. As suggested in issue #193 I tried changing the ultrasound_freq to 7 for one drone and to 8 for the other one. I am using this launch file. In one config the parameter ultrasound_freq ist set like this <param name="ultrasound_freq" value="8" /> and in the other one it is set to 7.

The problem that I am experiencing is that even though I am setting these values in my launch file they are not correctly set in data/config.ini of the AR drone. Whenever I try to set them to 8 the config.ini still reads 7. When I now edit the config.ini file and set ultrasound_freq = 8 manually, then start roslaunch again, the value will be overwritten to 7 while it should remain at 8.
The weird thing is that if I set my ultrasound_freq to 9 in my launch file and then start it, it will get overwritten in the config.ini and then reads ultrasound_freq = 9. When I change my launch file back to 8 again and then run it, the config still reads 9. It seems that all values are accepted except for 8. I even tried the example configs where ultrasound_freq is set to 8 but the config.ini still reads ultrasound_freq = 7.
The output when running roslaunch start.launch

$ roslaunch start.launch                                                  
... logging to /home/void/.ros/log/ad0ddd28-86d8-11e8-83c6-955de12afcbf/roslaunch-void-2627.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://void:46855/

SUMMARY
========

CLEAR PARAMETERS
 * /ardrone_autonomy/

PARAMETERS
 * /ardrone_autonomy/altitude_max: 1500
 * /ardrone_autonomy/altitude_min: 50
 * /ardrone_autonomy/bitrate: 4000
 * /ardrone_autonomy/control_vz_max: 2000
 * /ardrone_autonomy/control_yaw: 1.75
 * /ardrone_autonomy/cov/imu_av: [1.0, 0.0, 0.0, 0...
 * /ardrone_autonomy/cov/imu_la: [0.1, 0.0, 0.0, 0...
 * /ardrone_autonomy/cov/imu_or: [1.0, 0.0, 0.0, 0...
 * /ardrone_autonomy/euler_angle_max: 0.18
 * /ardrone_autonomy/looprate: 200
 * /ardrone_autonomy/max_bitrate: 4000
 * /ardrone_autonomy/navdata_demo: 0
 * /ardrone_autonomy/realtime_navdata: False
 * /ardrone_autonomy/realtime_video: True
 * /ardrone_autonomy/ultrasound_freq: 8
 * /rosdistro: kinetic
 * /rosversion: 1.12.13

NODES
  /
    ar_track_alvar_bottom (ar_track_alvar/findMarkerBundlesNoKinect)
    ar_track_alvar_front (ar_track_alvar/findMarkerBundlesNoKinect)
    ardrone_autonomy (ardrone_autonomy/ardrone_driver)

auto-starting new master
process[master]: started with pid [2640]
ROS_MASTER_URI=http://localhost:11311