No USB keyboard/mouse in win7 on a dualboot win10+win7, unless I hard-reboot

Running into a really weird issue with a dualboot windows 10/windows 7 machine, with win10 installed as main OS and win7 on a secondary drive, with windows 10's fastboot disabled. Win10 is set as default boot option, and the boot menu is set to a 15 second timeout.

If I boot the machine and let it time out or pick win10 explicitly, everything is fine.

However, if I boot the machine (or restart from win10) and then pick windows 7, my keyboard lights tu off the moment windows 7 starts up, and neither my mouse nor keyboard work. Unplugging and then plugging them back in does not help. However, if I hard-reset the machine, get to the boot loader, pick windows 7, and then tell windows 7 to start in normal mode (the hard reset causes it to offer the safe mode vs normal mode startup options), suddenly the keyboard and mouse work perfectly fine.

Has anyone run into this before, or do they know what might be the cause of this rather odd win7 behaviour?

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