I have a problem with my fans: sometime when waking up from hibernate, those fans connected to the motherboard sometimes spin for a bit then stop. They won't restart without a reboot.

Thankfully my AIO pump still runs when this happens. Idle temps go up from about 38 to about 45.

I've read that some Gigabyte motherboards (this is a Z790 Aero) have problems with PWM mode. But unfortunately with voltage mode forced in the bios, when it happens the fans still stay off, and the pump also turns off - taking idle temps to 70+.

Updating the bios doesn't fix it. Nothing else I've tried has worked. Short of installing silent fixed speed fans, is there a hardware way around this problem?

Maybe a dedicated, powered fan controller, with no fan stop? Ideally one with its own fan curve configuration, which persists even without running whatever Windows interface it has (system is dual boot). The closest I've found is something like Noctua's NA-FH1 into their NA-FC1.