This is a very old issue with APM Planner and I was hoping it would go away eventually, but maybe I'm not understanding how it's supposed to work.
When connecting to my copter using telem radio, I go to Initial Setup / Mandatory Hardware / Compass. I'm unable to enter something in the "minutes" slot, since it's disabled, despite Auto Dec being unticked. I have to select, then unselect Auto Dec to have the minutes section enabled again and then I'm seeing a negative value, see attached. Sometimes, I would enter a value here, fly, land, then reconnect and see a different value in the Degrees slot.
Is it just me struggling with these issues?