Installing a current version Debian Buster does not work because the default preseed finish template uses curl or wget to make the finish request to foreman via HTTP. Unfortunately current versions of the Buster installer do not install either of them so the request will not happen and foreman won't know that the host has finished building (apart from the fact that the installer stops and waits for user input because the script invocation fails).

Locking into the problem I couldn't find anything that would suggest that the default preseed templates ensure that either curl or wget are installed within the target. But the default finish template requires it to be present.

I would suggest to either include curl/wget in the "d-i pkgsel/include" list of the provisioning template (source:app/views/unattended/provisioning_templates/provision/preseed_default.erb) or to install it in the finish step if neither could be found. (Preference is on the first solution)