BlackTracer said:
Zeroing the display and not actually zeroing the barometer would be a useless and dangerous bug/feature. In fact I would not want that for the reasons you mention. I certainly hope it doesn't do that.
How would it be dangerous?
The altitude reading isn't used for landing. All that matters during landing is that the altitude is changing, not what the actual attitude is. When it stops changing, it's on the ground.
As for RTH altitude, just know that the altimeter is not accurate down to tens of feet and add some safety margin to your RTH altitude.
Also, see above. The altimeter is sensitive to temperature. Temperature is not a constant.
If you re-set the altitude on the ground after the Phantom has warmed up, once you take off it will cool down again and your altitude will be off again.
The best thing to do is leave it be, and when it cools off again after takeoff it will be more accurate.
But again, accuracy down to tens of feet (or meters) is not possible and not necessary in this application. Let it go and you'll be a much happier flyer.