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
Perhaps it stays at lower power level while no devices are connected, thus your body is enough to block it. But when you connect, it might increase the TX power so it suddenly works.
If you didn’t disable automatic connect, the client device should be sending probe requests for the hotspot’s SSID. Possibly the iPad also does this with lower TX power, so your iPhone doesn’t catch those requests either.
But that’s just a guess.
Also it’s possible there’s beam forming involved, but I am not sure if devices like smartphones can do that.