UPDATE AT BOTTOM
Still don’t know the exact root cause, but a ticket is currently open with Microsoft. It is somehow related to Bitlocker. Turning off Bitlocker before upgrading eliminates the problem. Microsoft is investigating, but since I’ve had so much trouble and time spent on this, I thought I’d post this in the hope I save some other sysadmin the headache.
UPDATE 2017-09-28: The problem certainly involves Computrace. Though turning off Bitlocker before upgrading definitely eliminates the problem, unenrolling and re-enrolling the device in Computrace fixes the problem as well, even after a reimage. A ticket is open with Absolute. I will update once I have further findings.