Out flying yesterday with Phantom 4 using out dated firmware.
To day I took the plunge and up dated my remote to version 1.9.3  and my Phantom 4 firmware to version 4.02000810.
then took it out for a short fly.
My return to home Altitude set at 35m
My Max Altitude set at 50m
did a couple of short flights with out problems, Then a short flight to check out my RTH, once RTH was activated I recieved notifiction of RTH at 35ms  the drone turned aroud in a hover and then began to climb,once 35ms was reached it just carried on climbing till it reached 50ms and just hovered their at which point I canceled the RTH and flew it back to me and landed it.
After serveral attempts with different Altitudes set and using the on screen RTH and Remote RTH  the same results were had.
I have up loaded the flight log any help would be most welcomed
https://app.airdata.com/flight/c89329978c11a9efa14dfe9651211cb5/GENERAL
flight log

It's best to use phantomhelp's site.

https://www.phantomhelp.com/LogViewer/NZATKTIFIB4JII2548RX/#

I don't see that you did anything wrong as far as straight piloting.  

I think it's a good idea to calibrate the IMU after a FW update.  See if that solves your problem.  

More importantly and unrelated to your RTH problem, you should never launch with anything less than a fully charged flight battery.  Otherwise you risk drop from the sky - it's happened to hundreds of people.

Good luck

On initiating RTH your Phantom turns to face toward home, ascends to your set RTH height but doesn't start coming home and climbs to 50 metres and gets stuck there.
You have your Max Altitude limit set at 50 metres.
Your had RTH Obstacle avoidance enabled.
The RTH direction is to the west.
What time of day was this incident?
Was the RTH toward a low, bright sun which the drone would have identified as an obstacle, tried to climb to avoid and become stuck at your Max Altitude limit?

Test your RTH when it's not trying to fly into the sun or turn off RTH OA.
There's no hint of any IMU problem and no need to recalibrate anything.

Labroides Posted at 12-30 15:17
On initiating RTH your Phantom turns to face toward home, ascends to your set RTH height but doesn't start coming home and climbs to 50 metres and gets stuck there.
You have your Max Altitude limit set at 50 metres.
Your had RTH Obstacle avoidance enabled.

Thank you for your input Labroides your  theory sounds good, yes my RTH would have been heading in to a bright low sun with OA enabled. I shall be testing your theory hopefully some time today and report back results.

bigal Posted at 12-30 23:10
Thank you for your input Labroides your  theory sounds good, yes my RTH would have been heading in to a bright low sun with OA enabled. I shall be testing your theory hopefully some time today and report back results.

Been and tested out Labroids theory.  Nice and sunny today so with OA enabled I did RTH  both into the sun and away from the sun had the same results just hovered at Max ALT.
turn off OA sencors  and tried a couple more RTH  and the Drone Returned to Home at the set RTH ALT.
Labroids was spot on.
I would have thought I would have got an Alert of some sort on my display but no. or even a reference to OA in the flight log but no.
Many thanks for your advice seems you know your stuff.
p.s I will try a RTH with OA enabled on a dull day see what happens.

bigal Posted at 2019-12-31 04:46
Been and tested out Labroids theory.  Nice and sunny today so with OA enabled I did RTH  both into the sun and away from the sun had the same results just hovered at Max ALT.
turn off OA sencors  and tried a couple more RTH  and the Drone Returned to Home at the set RTH ALT.
Labroids was spot on.

Well I tried RTH on a dull day with O/A enabled  results were failure just continued up to Max Alt and sat there.
With O/A disabled RTH was fine.
So what next?

bigal Posted at 1-12 03:40
Well I tried RTH on a dull day with O/A enabled  results were failure just continued up to Max Alt and sat there.
With O/A disabled RTH was fine.
So what next?

Was the RTH towards a low sun or was the sun nowhere near in front of your drone?
Do you really need obstacle avoidance anyway?

Labroides Posted at 1-12 18:10
Was the RTH towards a low sun or was the sun nowhere near in front of your drone?
Do you really need obstacle avoidance anyway?

No sun it was over cast. it would be nice to know that the O/A was working on RTH if only for piece of mind.

bigal Posted at 1-12 23:42
No sun it was over cast. it would be nice to know that the O/A was working on RTH if only for piece of mind.

I know it was overcast but I was asking if the sky in front of the drone was brighter than the rest of the sky or was it just dull.

Labroides Posted at 1-13 00:40
I know it was overcast but I was asking if the sky in front of the drone was brighter than the rest of the sky or was it just dull.

just as dull

bigal Posted at 1-13 01:37
just as dull

Presumably the OA didn't stop you from flying out.
Did you test in other directions?
Could you fly manually in the same direction as the RTH?

Labroides Posted at 1-13 01:43
Presumably the OA didn't stop you from flying out.
Did you test in other directions?
Could you fly manually in the same direction as the RTH?

O/A did not stop me from flying out
O/A did not stop flying in different directions
I could fly in all directions manually with O/A including RTH direction

bigal Posted at 1-13 08:11
O/A did not stop me from flying out
O/A did not stop flying in different directions
I could fly in all directions manually with O/A including RTH direction

Post flight data for the flight in question

Labroides Posted at 1-13 13:55
Post flight data for the flight in question

Flight Data no longer available,  but will upload my next flight logs once I get chance to get out and fly. Bad weather at the moment.