Update: I've verified this bug in a BIOS-mode boot in trusty, with a twist: BOTH the mei AND the mei_me modules must be blacklisted in order to cause a reboot to work. When both are blacklisted, rebooting also works when the system is booted in EFI mode -- I suspect a failure to blacklist mei_me caused the EFI-mode reboot failures noted in my previous report.
Update: I've verified this bug in a BIOS-mode boot in trusty, with a twist: BOTH the mei AND the mei_me modules must be blacklisted in order to cause a reboot to work. When both are blacklisted, rebooting also works when the system is booted in EFI mode -- I suspect a failure to blacklist mei_me caused the EFI-mode reboot failures noted in my previous report.