I am using iOS 15.5 currently in my Apple iPhone 12 Pro and I use it's Personal Wifi Hotspot frequently. While the hotspot functions flawlessly and never causes an issue, at times, when the internet is not actively used by the devices connected to the hotspot, iPhone stops the hotspot. It gets annoying you have to work discreetely as you would need to keep on enabling the hotspot over and over, and connect the device with the connection over and over as well. It just wastes a lot of time and definitely Apple must work on the issue. 

Currently the iPhone waits for 90 seconds for any data usage which is pretty small duration. Ideally it should at least 5 minutes, better yet it should be customizable but Apple never leaves anything for users to customize.