Comment by Borealid

If Microsoft wanted a backdoor, there is no need to hide it in the official Windows Recovery Environment image.

Just sign an alternate version of the recovery environment that doesn't bother displaying a login screen. Done - you can access any TPM-only Bitlocker setup freely. This is actually LESS risky than keeping the exploit in the publicly-available version of WinRE, because you don't have the risk of pesky security researchers finding your backdoor.

Hanlon's Razor and Occam's Razor both say this is probably a bug that lets you use some kind of early-boot filesystem-corruption-fixing code on the recovery image to break the login screen and leave the disk unlocked by accident. It deletes itself because it's, well, intended to be a filesystem fix log, and the log gets deleted when it's done being replayed so it doesn't happen a second time!