return to home on phantom 4 ascends to max height and hovers.

[Difficult-Airlines]

It's not that your Loss of Signal option was set to Hover, because that would only apply to loss of signal, not initiating RTH.
And it wouldn't make the Phantom climb.
One likely cause might be obstacle avoidance trying to climb over a low, bright sun that it sees as an obstacle.
You need to post flight data to confirm if that was what happened.

I had the same thing happen to me today, and the Phantom was indeed facing a low Norwegian winter sun.
Thanks for the tip!