It appears that the binaries for 4.2.3 for s390x on Ubuntu 18.04 are missing. If you take a look here, https://repo.mongodb.org/apt/ubuntu/dists/bionic/mongodb-org/4.2/multiverse/binary-s390x/, the latest binaries look like 4.2.2.
The lack of binaries is affecting the official mongodb Docker builds, https://github.com/docker-library/mongo/issues/384.
Are the binaries supposed to be uploaded? Or is support for s390x on Ubuntu 18.04 dropped?