When verifying signature signed by ed448 subkey, we see following in 2.3.2:
gpg: Signature made Wed Sep 29 10:12:36 2021 JST gpg: using EDDSA key 3F17746E8AE1BFE6FFCD698DBC7FE1DF1488327F9FE29A1F34CBA0CA4731DBA4 gpg: issuer "k@gniibe.org" gpg: WARNING: signing subkey 3F17746E8AE1BFE6 is not cross-certified gpg: please see https://gnupg.org/faq/subkey-cross-certify.html for more information gpg: Can't check signature: General error
This is due to v5 packet handling in merge_selfsigs_subkey.