Auto Linking Previous Tickets when stated in copy of email

[Service API]

Hoping someone can help here.

Previously when on the server platform of Jira we were able to send an email into our service desk and quote a previous ticket in the copy of the email. This would automatically link that ticket to the newly created one.

Since moving to Data centre this no longer works and the automatic link is not created.

This may have been working with an app or automated process but cannot for the life of me locate how where this was setup.

Any help with this would be appreciated.