Comment 2 for bug 1642582

I agree that there is a conflict, however I cannot reproduce it the way it is specified in the bug description, out of the box. From archive Contents file:

etc/bash_completion.d/mtx admin/mtx
usr/share/bash-completion/completions/mtx shells/bash-completion

One ships the completion file in the new location, whilst the other ships it in the old location.

The two should not create a dpkg conflict. And they do not, in a clean Ubuntu Xenial chroot (e.g. $ mk-sbuild xenial; schroot -u root -c xenial-s390x; apt update; apt install bash-completion mtx)

From the affected system, can you please provide output of:
$ ls -latr /etc/ | grep bash

to verify that e.g. /etc/bash_completion.d is not a symlink to usr/share/bash-completion/completions/ on your system.

Nonetheless, I will open a new bug report about this completion conflict between these two packages in Debian.