Yes the mails we receive have properly mime encoded headers.
But let's say some agency sends an email with ancient illegal raw big5
headers:
$ iconv -f big5 headers.txt
Subject: 內政部部長電子信箱受理通知編號:20210108003         
From: "內政部部長電子信箱" <returnecss@minister.moi.gov.tw>

Well, on the first day, things work fine in gnus.

But on the second day upon reopening gnus, the headers have "degraded":