Sub-tasks created inside a custom standard issue type appear as 'issues without epic' when using swimlanes based on epics on a board

Summary
Sub-tasks created inside a custom standard issue type appear as 'issues without epic' when using swimlanes based on epics on a board

Environment

Steps to Reproduce

1. create an epic and assign it to user1
2. create a custom standard issue type inside the epic and assign it to user1
3. create a sub-task inside the custom standard issue type and assign it to user2
4. go to the project board and change its settings to show the issues using swimlanes based on epic
5. go to the board and use the filter 'Assignee' and select the assignee of the sub-task (user2)

Expected Results
The sub-task will appear under the respective epic

Actual Results
The sub-task will appear at the end of the board as 'issues without epic'

Workaround
No workaround at the moment