I ran into this myself today after upgrading a machine to bionic.. 

two copies of it running at once.. both stuck on host.
If I execute a new 'host' command it works, but the existing ones are stuck.l


root     14181  0.0  0.0   4628   868 ?        Ss   13:05   0:00   /bin/sh -c cat /run/systemd/resolve/stub-resolv.conf | /sbin/resolvconf -a systemd-resolved
root     14292  0.0  0.0   4520   752 ?        S    13:05   0:00     run-parts --arg=-a --arg=systemd-resolved /etc/resolvconf/update.d
root     14320  0.0  0.0   4520   748 ?        S    13:05   0:00       run-parts /etc/resolvconf/update-libc.d
root     14354  0.0  0.0   4628  1672 ?        S    13:05   0:00         /bin/sh /usr/lib/avahi/avahi-daemon-check-dns.sh
root     14607  0.0  0.0 187532  8380 ?        Sl   13:05   0:00           host -t soa local.

root     13775  0.0  0.0   4628   868 ?        Ss   13:05   0:00   /bin/sh -ec ifup --allow=hotplug eno1; ifup --allow=auto eno1;      if ifquery eno1
 >/dev/null; then ifquery --state eno1 >/dev/null; fi
root     13787  0.0  0.0   4592  1868 ?        S    13:05   0:00     ifup --allow=auto eno1
root     14179  0.0  0.0   4628   772 ?        S    13:05   0:00       /bin/sh -c /bin/run-parts --exit-on-error /etc/network/if-up.d
root     14182  0.0  0.0   4520   768 ?        S    13:05   0:00         /bin/run-parts --exit-on-error /etc/network/if-up.d
root     14183  0.0  0.0   4628   772 ?        S    13:05   0:00           /bin/sh /etc/network/if-up.d/000resolvconf
root     14461  0.0  0.0   4520   752 ?        S    13:05   0:00             run-parts --arg=-a --arg=eno1.inet /etc/resolvconf/update.d
root     14479  0.0  0.0   4520   728 ?        S    13:05   0:00               run-parts /etc/resolvconf/update-libc.d
root     14503  0.0  0.0   4628  1612 ?        S    13:05   0:00                 /bin/sh /usr/lib/avahi/avahi-daemon-check-dns.sh
root     14606  0.0  0.0 187532  8384 ?        Sl   13:05   0:00                   host -t soa local.


(gdb) t a a bt

Thread 4 (Thread 0x7f3231f00700 (LWP 14796)):
#0  0x00007f3237b06bb7 in epoll_wait (epfd=5, events=0x7f3238eb1010, maxevents=64, timeout=timeout@entry=-1)
    at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
#1  0x00007f323804773b in watcher (uap=0x7f3238eb0010) at ../../../../lib/isc/unix/socket.c:4280
#2  0x00007f3237ddd6db in start_thread (arg=0x7f3231f00700) at pthread_create.c:463
#3  0x00007f3237b0688f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 3 (Thread 0x7f3232701700 (LWP 14794)):
#0  0x00007f3237de39f3 in futex_wait_cancelable (private=<optimised out>, expected=0, futex_word=0x7f3238eae0a4)
    at ../sysdeps/unix/sysv/linux/futex-internal.h:88
#1  __pthread_cond_wait_common (abstime=0x0, mutex=0x7f3238eae028, cond=0x7f3238eae078) at pthread_cond_wait.c:502
#2  __pthread_cond_wait (cond=0x7f3238eae078, mutex=mutex@entry=0x7f3238eae028) at pthread_cond_wait.c:655
#3  0x00007f3238039370 in run (uap=0x7f3238eae010) at ../../../lib/isc/timer.c:808
#4  0x00007f3237ddd6db in start_thread (arg=0x7f3232701700) at pthread_create.c:463
#5  0x00007f3237b0688f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 2 (Thread 0x7f3232f02700 (LWP 14791)):
#0  0x00007f3237de39f3 in futex_wait_cancelable (private=<optimised out>, expected=0, futex_word=0x7f3238eac0cc)
    at ../sysdeps/unix/sysv/linux/futex-internal.h:88
#1  __pthread_cond_wait_common (abstime=0x0, mutex=0x7f3238eac028, cond=0x7f3238eac0a0) at pthread_cond_wait.c:502
#2  __pthread_cond_wait (cond=cond@entry=0x7f3238eac0a0, mutex=mutex@entry=0x7f3238eac028) at pthread_cond_wait.c:655
#3  0x00007f32380329e0 in dispatch (manager=0x7f3238eac010) at ../../../lib/isc/task.c:1086
#4  run (uap=0x7f3238eac010) at ../../../lib/isc/task.c:1312
#5  0x00007f3237ddd6db in start_thread (arg=0x7f3232f02700) at pthread_create.c:463
#6  0x00007f3237b0688f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 1 (Thread 0x7f3238eeabc0 (LWP 14607)):
#0  0x00007f3237a24236 in __GI___sigsuspend (set=set@entry=0x7ffd71576aa0) at ../sysdeps/unix/sysv/linux/sigsuspend.c:26
#1  0x00007f323803c4d1 in isc__app_ctxrun (ctx0=ctx0@entry=0x7f32382718e0 <isc_g_appctx>) at ../../../../lib/isc/unix/app.c:721
#2  0x00007f323803c7cc in isc__app_run () at ../../../../lib/isc/unix/app.c:754
#3  0x00007f323803d090 in isc_app_run () at ../../../../lib/isc/unix/../app_api.c:198
#4  0x0000563cebce752a in main (argc=4, argv=0x7ffd71576d08) at ../../../bin/dig/host.c:906
(gdb)