In the VTB the @mention functionality does not work when the user clicks on comment in the activity stream
Kingston, London, Madrid
This is expected behavior as the @mention functionality works only in the below scenario.
1. On a form, in the activity formatter (UI16 only).
2. In a list, in the activity stream for a specific record.
3. On a Visual Task Board card, in the card details.
4. In a Connect record conversation.
This is expected behavior and there is an enhancement request for this already.
For more information, please go through below documentation.