PVE 9.1.1 with vm win2008R2 the memory usage show correctly after install balloon service
https://pve.proxmox.com/wiki/Dynamic_Memory_Management#Enable_Auto-Ballooning_on_Windows_2008r2
win2008r2 with virtio version 173
I have the same problem.
vm (KVM) lost network.
Not use the kernel 2.6.32-45-pve !!!
VM is centos 6.x or windows 2008 R2
I will provide more info if I can.