https://support.atlassian.com/jira-work-management/docs/create-and-edit-issues-and-subtasks-on-the-board/#View-subtasks-on-your-board
 

Currently, the board does not support filtering by subtask. You can only view subtasks that are nested under their parent.

Nowadays, the board filter option in JWM Projects shows the subtask as item to help filter issues, however, this option is not working as expected and there is an effort to remove the subtask from the filter list. In fact, it would import to fix the current behavior and not remove the item.