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If the fan spins 5V must of course be present at some point. Heck ALL_SYS_PWRGD needs to be present for the fan the even spin. So the board is doing a lot more than you just reported.
Is CPU Vcore coming on when the fan is spinning?
Could be anything. CPU is not talking to U7400 to enable it. Inspect board for missing/damaged parts. R7402 is a common suspect, but as I said could be anything. BIOS is also possible.....anything.