Stories are disappearing from the backlog (in classic) when moved from a next-gen project

Issue Summary

Stories are missing on backlog and some epics are displayed as swimlanes (if the board swimlane is set to Story)

Environment

Jira Software Cloud

Steps to Reproduce

1. Create an Epic and a Story on a Next-gen board
2. Add the Story to the Epic
3. Move both issues to a classic project with Scrum board
4. Open the Scrum board and the Story will be missing since it's recognized as sub-task

OR

1. Create an Epic and a Story on a Next-gen board
2. Add the Story to the Epic
3. Move both issues to a classic project with Scrum board
4. Create a sprint on classic board
5. Add the issues to the sprint and start it
6. Set the board swimlane to Story
7. Open the Scrum board, go to active sprint and the Epic will be the swimlane

Expected Results

Story is displayed on backlog

Actual Results

Issues are missing on board and an issue.field.issueparentassociation log can be found in the History of the affected Issues.

Workaround

Contact Support.