I am still running version 13 but from what I can tell, this is still working this way in current version.
When doing a slash command in Mattermost, posting to a GitLab project Mattermost Slash command integration service, GitLab appears to returns the data to Mattermost (such as the issue created, comment added, etc.) execution as “ephemeral” (or not set which ephemeral is default). Thus it is shown as “visible to user only”.
This defeats a little bit the purpose of ChatOps where all the actions done by a person are visible to the other members so as to share the knowledge. A command executed from the chat should be visible to the other members as well as the results.
See Slash output "Only visible to you" - #9 by etlweather - Integrations and Apps - Mattermost Discussion Forums for some info on this.
I am not sure if it is possible to change this somehow of it is a code change in GitLab that is needed. I couldn’t find a configuration point for this.