I have very similar issue:
this part of preseed fails (did not fail in stretch), with error message - Could not reach repository (options are retry, modify or ignore - or something similar) (in syslog it says - Repository not signed)
d-i mirror/country string manual
d-i mirror/http/hostname string 10.88.0.1
d-i mirror/http/directory string /debian-10-amd64
d-i mirror/http/proxy string
what helps little bit is to ignore the above error manually, which will just comment above repository in sources.list (# deb
http://10.88.0.1/debian-10-amd64 buster main) and following line in preseed:
the installation finishes fine then