(I checked incus admin os system network show but I don’t see a matching key; I tried adding hostname: myservername right under config: but it was silently ignored.)
What made me notice the random hostname was enabling the Tailscale service, as Tailscale uses it as the machine name in the config paneL
I looked through the config options in the webUI, the docs and the forums but haven’t found a way to set the hostname (I’ve changed it manually on Tailscale’s side, but it made me wonder).