Hmmmm... weird.
I just cloned my debian 12 template that auto-installs qemu-guest-agent via Cloud-Init and it has no issues at all.
To start, did you enable the qemu-guest-agent? Since by default it is disabled.
systemctl enable qemu-guest-agent
systemctl start qemu-guest-agent
I see that you forcefully exited the systemctl start command, was that due to it hanging for a long time? (Since once it is started or errors out it should exit on its own.)
And if so how long did you wait for it before forcefully exiting the systemctl start command?
What do the journal logs say?
Do you use the latest Cloud-Image from
https://cloud.debian.org/images/cloud/bookworm/latest/ or an older cloud-image?
And if you use an older version then please try the latest version first.
Lastly how did you install QEMU-guest-agent?
Did you install it yourself via APT or did you use Cloud-Init?