Personal Hotspot not staying Connected I'm not sure if I have an issue or if what I am seeing is by Apple-design.  I have an iPad Pro (11.4) that I have the Personal Hotspot switched on.  


My iPad Pro is active and screen on.
I connect my iPhoneX (11.4) to the iPad Hotspot.
Everything works fine. 
I sat working on laptop and then the iPhone X and iPad screen locked.  After X minutes I then unlock iPhone to check email etc.. 
The iPhone X has disconnected from the hotspot and is back to using the cellular plan on my iPhone.  If I go to Settings -> Wi-Fi then my iPad is not listed. 
To make the hotspot reconnect I have to go back to my iPad and toggle the Personal HotSpot switch (On -> Off -> On).


Is this by design or not?  Should the iPhone remain connected if the devices screen lock?
After further experimentation I found that if I create a Bluetooth connection (pair) between iPhone/iPad it stays connected throughout screen locks etc.