Unable to display owner of Test cases in JIRA JQL

Hi

Our customer is using Zephyr Scale in their Jira instance. 
They track their test cases via Jira issues and Jira custom fields and in one of the projects 1 out many users is not found the following JQL (see example below). 
Everyone else in the project is found when using the query. 
I've made sure that the user is the right group and has permissions in the JIRA project, also making sure relevant groups is added in the Zephyr project settings. 

project = "PROJECT KEY" and issue in coveredByTestCasesHavingField(owner, username)

Have this occurred before?
Any tips or ideas ? 

Kind Regards,