The problem happened to me this morning as well. I woke this morning, unplugged my iPhone XR from my adapter and then found my phone showed "iPhone Unavailable".
I went to Apple store Taipei 101 and consulted an assistant directly, but unfortunately, he knew nothing about the issue and told me that the only thing I can do is to reset my iPhone to factory default. However, I never backed up my iPhone data, so I am unable to restore my data.
I have numerous precious data in my iPhone and am eager to know know how I can fix this issue without resetting my phone to factory default.
Really hope to get the solution from Apple or anyone in community.