Hello

I have found another possible cause for nginx to ignore default server
I have encountered it just now, and it is not your case, but maybe it would be helpful for someone

At example:

server {
listen 80 default_server;
server_name default.domain.dom;
................
}

server {
listen <particular.ip>:80;
server_name some.server.domain.dom;
..................
}

In this case, listening schema for the default server is _not_the_same_ as for another server.
Considering the fact that we have described a server with the particular ip address, our default server for that ip (although it is listening on all IPs) would be ignored, and all queries to that particular ip, even with "Host: default.domain.dom", would be directed to "some.server.domain.dom" as it becomes a default (first-described) server for this listening schema.
We have to explicitly describe "listen <particular.ip>:80" on the default server to enable it for this schema.

WBW, valinor