I can confirm with curl that I receive different headers from the server, depending on whether or not I send my login cookie. (I obfuscated the value of my cookie for pasting.) This agrees with what I see in Chrome's developer tools network inspection, which lists headers without a charset in anonymous/incognito, vs headers with charset in my tab where I'm logged in.
C:\Users\Peter>curl --head https://perlmonks.org
HTTP/1.1 200 OK
Date: Mon, 22 Mar 2021 22:14:50 GMT
Server: Apache
Upgrade: h2,h2c
Connection: Upgrade
Last-Modified: Mon, 22 Mar 2021 22:12:10 GMT
ETag: "ee70-5be275b693fc1"
Accept-Ranges: bytes
Content-Length: 61040
Content-Type: text/html
C:\Users\Peter>curl --cookie "userpass=XXXXXXXXXXXXXXXXXXXXXXXXXXXX" -
+-head https://perlmonks.org
HTTP/1.1 200 OK
Date: Mon, 22 Mar 2021 22:14:26 GMT
Server: Apache
Upgrade: h2,h2c
Connection: Upgrade
Content-Type: text/html; charset=Windows-1252
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edit: sorry, first time I posted, I misread the statement, so I deleted one sentence where I commented based on my misreading.