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.