The Gitlab commits are usually displayed under “mentioned on” section in JIRA which is not working anymore. I am unsure if the issue occured post JIRA upgrade to 9.4.4. While the old commits continue to be displayed under “mentioned on” section on a JIRA issue, the new commits/merges are not shown.
Depends on what’s your integration between Jira and GitLab. The integration is not automatic and someone actually had to set it up. There are GitLab docs available for the integration setup.