I’m not sure if documentation is accurate regarding autoinstall/preseed (apb)…
I tried, but failed.
https://askubuntu.com/questions/1238696/preseed-autoinstall-does-not-work-on-20-04
Which documentation are you referring to?
You also mention “using cubic on ubuntu 20.04 update to generate the iso”. I have never heard of this tool before, presumably this is https://launchpad.net/cubic which is hosted in Launchpad but has never been submitted for inclusion as a package in Ubuntu and the Ubuntu team cannot vouch for it.