apt seems to be ignoring Signed-By

$ cat /etc/apt/sources.list.d/yuuki-deb.sources
Types: deb
URIs: http://yuuki-deb.x86.men/ 
Suites: bullseye
Components: main
Signed-By: /usr/share/keyrings/yuuki-deb.gpg
Enabled: yes

$ ls -l /usr/share/keyrings/yuuki-deb.gpg
-rw-r--r-- 1 root root 433 Sep  7 20:23 /usr/share/keyrings/yuuki-deb.gpg

$ gpg --show-keys /usr/share/keyrings/yuuki-deb.gpg
pub   ed25519 2020-03-03 [SCA]
      A9BBA31152359AE080A1DF851F331533ABCDEEA3
uid                      AviSynth+ Yuuki Debian Repository 

# apt update
-*- snip -*-
Err:4 http://yuuki-deb.x86.men  bullseye InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F331533ABCDEEA3
-*- snip -*-