This has happened a couple times.  I boot up the MP........the X says it is disconnected and a warning states that the ultrasonic system needs service and send it to DJI.
After several attempts to clear it I finally rebooted the X and all is well.

I have an X, and have never experienced anything like that.  I have had a few disconnects when trying to use the cable that goes into the mini usb on the side, but only because the connection to the phone is still a little awkward.  (Had same issues on my Pro, and just gave up and used a longer cable that wasn't fed through the folding arms)

Hi there.
Since the controller of the Mavic Pro has two modes to connect, you can change another USB port to connect your iPhone X and your RC. Please note that if you want to use the port below the RC, you should unplug the cable on the left first. If the error comes up many times, you may kindly provide us with the screenshot of the error message at your convenience. We will check and help.
Thank you in advance.