In order to get my iPad to connect to my phone I have to take the phone out of my back pocket. I don’t have to unlock the phone or anything. Just take it out. If my phone is in my back pocket the iPad can’t detect the phone. Once the two are connected I can put the phone back into my bank pocket, and use the phone as a hotspot
Pretty sure it’s powering on the screen that does it. Even when sitting on a table I can’t hot spot till I pick up the phone, the movement of which powers on the screen. I thought it was getting a Face ID that did it, but if you’re saying you don’t have to unlock then it’s possible it’s wanting just enough interaction to know the connection is intentional so you don’t accidentally use up your data cap.