We've ran into this issue before in codfw (T296856):
These servers are failing to install buster with the outdated firmware on the system. Following the reimage over the serial console I can see that the d-i image and the initrd are being served (and it's also showing up in the logs of the install* server), but after that the system reboots into the installed system. These can only be installed with buster after firmware has been updated. VMs need to be drained from instances, so I'm listing the servers and when they have been freed of instances (we can't free up too many at a time) and please tick them off when firmware was updated. It'll be 18 servers in total.