This might indeed be a bug, or have some specific cause not known to me. Maybe some other community member will chime in that has (had) this experience aswell, and knows the solution.
The only reason i could think of is before first login when powering on the device, contacts are still encrypted. So when u receive a call then before first login, only the number would be displayed. But i assume this isn't the case here.
Maybe, if a bug, the issue lays there somewhere
Perhaps consider making a github issue if nobody chimes in with this experience or explanation after a day or 2.