When using @ (at) mention in HR Cases in the Additional Comments or Work notes field, the information (most notably the HR Case number) does not appear in the corresponding e-mail.
The @ (at) mention functionality works fine in all other ITSM applications.
This issue is a documented Product Defect, PRB1388948, and is fixed in the Paris family release.
For a workaround, please see the aforementioned PRB, or raise a case with Support mentioning the PRB # and this KB #. A workaround will be provided on-demand.