To still have the issue after a factory reset, I'd say its a hardware issue with the main board in how it doesn't go back to soft power state. Maybe a BIOS flash might fix this. But before you do that perform a RAM test with MEMTEST86 running it for at least 4 full tests as for I can seen RAM issues at shutdown cause this sort of behavior when it writes to disk before it completes shutdown and a memory address has an issue causing the shutdown to never fully complete causing a black screen and system still running. Holding power button forcing shutdown still works when the system had this issue via a forced hard shutdown.
