NGINX proxy_pass host resolution inconsistent with IP6

I'm using NGINX in a simple reverse proxy scenario, usually with a variable to define host names so that it won't fail to start if the host is unavailable.

Here's what I find: With this configuration traffic is proxied, but the pages don't render completely - it looks as if some of the .js libraries aren't loading.

    resolver 192.168.86.1 valid=30s ipv6=off;
    location /cam/ {
               set $motion01 motion01;
               proxy_set_header Host $host;
               proxy_pass http://$motion01.lan/;

If I remove the ipv6=off like this, then I get 502 bad gateway, and the error shows "host not resolved":

    resolver 192.168.86.1 valid=30s;
    location /cam/ {
               set $motion01 motion01;
               proxy_set_header Host $host;
               proxy_pass http://$motion01.lan/;

But, if I change to use a literal host name rather than a variable, it works perfectly:

               set $motion01 motion01;
               proxy_set_header Host $host;
               #proxy_pass http://$motion01.lan/;
               proxy_pass http://motion01.lan/;

I don't want to use a literal host name as nginx won't start if the host isn't available at startup time, but I can't figure out what is up with this configuration.

I've tried with IP6 enabled and disabled on the server, and with including and excluding the

listen [::]:443 ssl; 

directive, all to no avail. This wasn't an issue with the earlier version of nginx (can't remember which I originally used), but is now with 1.14.2.2.

Any and all suggestions welcome!