Why are domain names converted in output html?

Question

IIS

There was an amazing glitch: in the information (html code) given by the server, domain names began to be substituted. Can anyone suggest what it could be?
Example:
I create a test.html file
In the html body of the file I write the text "Test address mysite.ru/myurl?id=1 " I upload it
to the server, the site displays the text as "Test address http://promo.mysite.ru=1 "
It differs it is the original Html, not the visual display.
That is, it turns out that the Html file goes through some kind of filter that replaces its contents at the output (that is, the option with Js disappears).
It's as if the conversion happens according to some kind of rewrite rule, but only in the html body, and not in the address bar.
On the local version everything is fine.
I did not change any settings, but shortly before that, the admin was fiddling with firewalls and dns.
At what stage is it worth looking for the source of the transformation? iis, firewall?