I've tried multiple 3.5mm headphones -- none of them work unless I have them plugged in before boot. (i.e. same problem as the OP). All of these headphones work on my other devices flawlessly, but they don't on my new $1600 Dell XPS 13 2in1 7390.
Tried killing the MaxxAudio app in Task Manager and starting it again to have it recognize my earphone, but it still doesn't.
Tried using a 3.5mm to USB C dongle, and then plugging it into the USB C port -- the earphones still dont work (They work on other devices with the dongle too)
Live booted into Ubuntu and the port and earphones work -- so the hardware is alright