Commit https://github.com/vyos/vyos-1x/commit/84b7ade286e4022e62684237246cd04b9d37b5db seems to have introduced a regression regarding the domain-search option.
Previously, multiple values were expressed in dhcpd.conf like so:
domain-search "domain1", "domain2";
domain-search "domain1, domain2";
which seems to cause isc-dhcp-client to ignore the value.
Maybe this also affects other options, haven't checked so far.