JQL function "parentEpic" does not work with team-managed (formerly next-gen) projects, "parent" should be used instead. However, there is a significant difference between those functions: "parentEpic" shows the full tree under an epic (including the epic itself in addition to issues down to sub-task of a story of an epic), but "parent" shows only the direct child issues of an epic.

It would be good to have the functionality of parentEpic in team-managed (formerly next-gen) projects.