I'm still having the same problem in Mission Planner when I enter the Magnetic Declination in the Planner, if I enter a value "-3.6", "-3,6", etc, the value it assumes will be "-36", so it's not reading the decimal separator. I'm having this problem in all revisions from this version (my keyboard is in portuguese, decimal separator is defined as ", (comma)"). I always go to the CLI and set it up there, since in CLI works well (setup -> declination -3.6). As this is not a big deal, I haven't mentioned earlier, however, lot's of folks could be complaining about compass not working well or not showing heading correctlly, and it could be as simple as this.
They might be entering a decimal value for instance 2.2, and the declination would be set to 22. I recomend everyone checks after setting it.
I changed the decimal separator to ". (dot)" and now it works fine. So if your decimal separator in windows does not match the Mission Planner, it will not assume it. Go to regional settings and use the dot for decimal separation and comma for grouping.