Issue Summary
Currently, actions via the customer portal won't result in any Jira mention notification emails.
As detailed in documentation related to Add participants by mentioning them via the customer portal

For example: Jira will not send mention notification emails for a Mentioned User if automation based on the 'Issue created' action is triggered from the Customer Portal.
Note
There is currently a bug where in certain circumstances if 'Batching email notifications' is enabled, Jira will send mention notification emails on issues created from the customer portal. However this behaviour is not to be expected.