We are able to reproduce this issue at will in Ubuntu Bionic's installer (not identical to Debian's, but code-wise in this path the same). While quite a while after the last update from Philipp, we tested the patch (netcfg_dhcp_domain.patch) after updating it to avoid a compilation issue, we found it did fix the problem for us.
I am not sure if I can get Debian into our infrastructure to test explicitly, but I will work on it; at the same time, the code change seems straightforward. Thanks! -Nish
