So apparently USB port to PS/2 emulator works as the following.
In this connection we have some problems with the automatic emulation of USB keyboard and mouse to PS/2 performed in the BIOS. Basically the BIOS emulates PS/2 keyboard and mouse devices for operating systems that wish to use this instead of USB keyboard and mouse. It does this (as far as I understand) by inserting itself in all PCI USB hubs found on the system. In order to ensure the OS does not interfere with this emulation, the used resources are reported via ACPI RMRR records.
Lenovo forums
Although this still is no answer to your issue, try what reV said and additionally check your motherboard drivers + update BIOS if update has been released.
Additionally if I remember correctly you had a Gigabyte motherboard. Try the following if applicable.
Turn on/off a Lock on the USB keyboard, the PS/2 keyboard's Lock state also changes. The PS/2 keyboard is seen in Win10's device manager, drivers are reportedly up-to-date. Additionally, if I unplug/re-plug the PS/2 keyboard, it starts working (required after every reboot)
Superuser forums