Why doesn't the domain open to test the site and redirect to the combat one?

Question

System administration

We have a problem with subdomains. There is a site in https, which has a test domain (maintest.domen.ru) and a combat one (domen.ru). There was a problem that if we
open the test domain maintest.domen.ru, it automatically transfers to the combat domain (domen.ru). The admins, who are engaged in technical support of the site, wrote about it.
But either I'm not explaining something properly, or they're stupid.

I quote the correspondence:

From: I - 2017-03-28 01:45:37
when you open maintest.domen.ru, domen.ru opens, i.e. test does not work

From: Site admin - 2017-03-28 01:47:38
Hello. We are opening.

From: Me - 2017-03-28 01:49:57
if I want to open maintest.domen.ru it should open! now to the main combat throws

From: Me - 2017-03-28 01:50:41
although this works kzn.maintest.domen.ru for some reason

From: Me - 2017-03-28 01:54:05
if you open https://domen.ru later maintest.domen.ru then the combat engine will open

From: Site admin - 2017-03-28 02:15:17
This is a feature of https, the browser "remembers" the https connection and connects to it. There are no problems from the server side

From: I - 2017-03-28 02:17:17
so maintest.domen.ru and domen.ru are different domains

From: I - 2017-03-28 02:19:31
and just like him can behave this way if these are different domains! you apparently do not understand the problem! that's impossible! otherwise it would always open any last
subdomain

From: Site admin - 2017-03-28 02:22:40
From my side, the domain opens maintest.domen.ru

Please, judge us, are the admins really right and this can be or are they stupid ??

And please suggest a solution.

ps checks for Strict Transport Security (HSTS) No

Answer 1

maintest.domen.ru and domen.ru are different addresses, but they can lead to one site.
It all depends on:
1. how the DNS server is configured
2. how the host is configured
On the first point, you can check what your real domain is?
ps https has nothing to do with it, the admin is disingenuous

Answer 2

Do you have test and production sites on the same IP? Then everything is logical.
Your production site is available via https (port 443), and the test site via http (port 80). When you go to the combat domain, and then try to switch to the test one, you use the url http s ://maintest.domen.ru
And since the test domain does not have https, you have a combat one on port 443. Specify the full url for the test domain.