For a fresh install of Ubuntu 18.04 I found that libnss-resolve needed to be 
installed to fix various systemd-resolvd errors (with a setup like you 
describe, gateway does DHCP for a local net and DNS).
Your case may be different because you seem to have a null domain.  My ISP sets 
up a line like "search blah.blah.isp.net" which becomes the domain for nslookup 
of plain names (without an ending period).  When the ending period is used on 
the plain machine name, then just the name is returned without the period or 
domain and the address.
  Another problem may be that upgrades from 16.04 may result in a different 
systemd-resolvd setup.
The standard I assume is /etc/resolv.conf is a link to 
/run/systemd/resolve/stub-resolv.conf, which contains nameserver 127.0.0.53, 
options edns0, and search blah.blah.isp.net.  There is another file 
/run/systemd/resolve/resolv.conf which contains the gateway instead of 
127.0.0.53.  If you switch the /etc/resolv.conf link to this file, you cut 
systemd-resolvd out of the loop, fixing some problems but maybe causing others.
  The libnss-resolve package changes the hosts line in /etc/nsswitch.conf to:
hosts:          files mdns4_minimal [NOTFOUND=return] resolve [!UNAVAIL=return] 
dns myhostname
which fixes all problems I have noticed (including the dns failures when 
running with a reduced function set after an NXDOMAIN error (see syslog)).

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https://bugs.launchpad.net/bugs/1777579

Title:
  18.04 Desktop LTS DNS behavior (systemd-resolved)

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