Please find below video for your reference.
I got the latest firmware and app on my iOS devices.
Same result.
https://youtu.be/0ZO4T586lOs

Check the RTH section of your manual and it will tell you that if activated closer than 20 metres from the homepoint, your Phantom will simply land where it is.

No-one knows the reason for this but that's the way it's programmed.
The only people that usually encounter this are timid new flyers because there's really no reason anyone would be using RTH when the Phantom is less than 20 metres away.

Go to a big, open area, fly your Phantom 100 ft or more away and experiment with RTH to see how it works and learn how to initiate RTH and how to cancel and resume control.

Sorry for bother, will it apply to Mavic as well by this setting?
People who take off in a very limited area (like jetty, top of mountin etc) will need to pre check the drone before a long/real fly, if there are any problems with your remote or interruptions during this check your drone will probably face an incident...
Btw, thank you for letting me know this settings...

skparco Posted at 2017-12-26 22:59
Sorry for bother, will it apply to Mavic as well by this setting?
People who take off in a very limited area (like jetty, top of mountin etc) will need to pre check the drone before a long/real fly, if there are any problems with your remote or interruptions during this check your drone will probably face an incident...
Btw, thank you for letting me know this settings...

Yes .. the Mavic will be the same.
If you allowed the drone to RTH and auto-land on a jetty or close to the water or a clif, it could bring tears so it's a good idea to make sure your launch point is clear of potential problems like that.
But you don't need to let it auto-land.
As long as you have signal, you can easily cancel RTH and resume control and land where you choose.

Got the answer from Mavic manual, 5 meters