This article is a bit confusing as this IS what I want to do.
https://www.adaptavist.com/blog/top-10-most-commonly-used-jira-query-language-functions#epicsOf
Example: issueFunction in issuesInEpics ("project=JRA and status = 'in progress'")
Example for ‘issuesinEpics’
As a project manager, you might want to look at open epics and then at issues in specific statuses to see how many open, in progress, or other issues’ statues are in the current project. Specifically, you may want to see issues in the open epics that are also open, so you can gauge the work your team has yet to start.
For version 1.0, I want all open issues in open Epics.
The result I get is all issues in the open epics regardless of fix version, including the closed tickets.