so openssh on musl linux (alpine, openwrt, buildroot, yocto, and a lot of the docker images you've pulled) has had a little problem since 2016.

accounts that should never take a password at all (root, your key-only deploy user, ldap users, the default user on alpine's aws and azure images) take any password longer than 256 characters.

https://sshhole.com/

Apply patches ASAP and see https://alpinelinux.org/posts/2026-11-05-openssh-password-authentication-bypass/