It sounds like you're running up against this bug -https://jira.atlassian.com/browse/GHS-11065
I would recommend watching that for any updates on the progress, as well as voting/commenting to provide feedback to the dev team.
With the latest upgrade of Jira we have noticed that this feature is not working anymore for customers who do not have Jira Agile enabled. The "Issues in Epic" feature is provided by Jira Agile and is not a default feature for Jira.
Why this was working in the past is unclear and currently under further investigation. There is a bug report already opened for this problem: https://jira.atlassian.com/browse/JRA-39616
Unfortunately there is no workaround at this point. Please feel free to add your comments and vote for it to make it more noticeable to our developers. Also refer to Implementation of Bug Fixing Policy for more information on Atlassian's approach to the development of these bugs fixing.
Please let us know if we can help you any further.
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