The Suggestion

Currently, Jira editor fetches the user's mention from the Site Administration, so let's assume that we have a user called Dummy User. Dummy, has a username of dummy.user under the Site Administration.

Once Dummy is mentioned in Confluence, he is mentioned based on his defined nickname, from the People Directory, so everyone calls him @Big Dummy (a nickname he defined himself) although, once he is mentioned in Jira, he's mention becomes dummy.user.

More examples

This is specially impactful for users that are provisioned via Okta, since users provisioned from there are created with an Unique Identifier from Okta, and in order to be mentioned, those users need to called out with the UUID, instead of their display names or emails.

Workaround

A workaround for this situation, is to provision the user, delete them on the Site Administration, and ask those users to log into the application, this will sync the user with the data from the database and allow Jira to fetch the user's mention based on their Display name, and/or email. Site administrators would need to reinvite users to the instance to sign up to see this workaround work for them