The information you're seeing comes from the QEMU Guest Agent, which is developed by QEMU project.
If you exclude "Proxmox" from your search and instead look up "QEMU Guest Agent pretty-name", you’ll find some relevant reference materials: QEMU Guest Agent Reference
It could be a QEMU Guest Agent API bug, but it's more likely that Windows natively stores/reports that particular value in a way that leads to this result.