Normally we default to acquiring dhcp lease, storing it, and using hostname from it.
When we do dhcp lease, we use predictable random clientid based on the interface name & machine-id.
We do not use MAC address as Client ID as used to be common in the past, but often enough leads to duplication / impersonation, or privacy leaks.
Can you boot interactive installer and switch to shell either using help menu, or by switchting to a spare tty?
From there you can check the network status with $ networkctl
and check the contents of leases in /run/systemd/netif/leases
You should there find state files, of leases that systemd-networkd-dhcp-clinet has received. Is there a hostname you expect there? or is there not?
If not, you can either troubleshoot your dhcp server (if you control it and can change it), or you can tweak the clinet.
In /etc/netplan
you can switch to MAC addressed based dhcp Client ID, which may cause dhcp server to behave differently, and issue a different lease with the expected hostname, which then should be picked up. Specifically from https://netplan.io/reference#common-properties-for-all-device-types there is dhcp-identifier: mac
option that will change the behaviour of the systemd-networkd-dhcp-client. After changing that, you can try to do $ netplan apply, and like bring the device down / up, to request a new lease.
Maybe then you can inspect leases in /run again, it is what you expect, and then you could include the netplan config with that setting in your autoinstall.yaml.
Or try to fix your dhcp-server to recognise the client-ids you want.
This is more of netplan question however, I do not believe there is anything installer specific about this issue.