1. /board/{boardid}/issue API returns incorrect issues:

Steps to reproduce the bug:

• Create a new next-gen type project
• Turn on the “Backlog” at the project features, but don’t turn on the “Sprints”
• Create 3 issues in the backlog
• Add only the first issue to the board (let’s say its id is 2)
• Call this REST API endpoint: https://<JIRA_BASE_URL>/rest/agile/1.0/board/2/issue
  https://developer.atlassian.com/cloud/jira/software/rest/#api-rest-agile-1-0-board-boardId-issue-get

Expected Result

JSON result contains only the first issue.

Actual Result

JSON result contains all 3 issues.

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2. /backlog API not returning resolved issues

Well explained in https://community.developer.atlassian.com/t/jira-software-cloud-get-issues-for-board-rest-api-does-not-return-the-correct-issues-for-boards-of-next-gen-projects/32951/3

Reference:
https://community.developer.atlassian.com/t/jira-software-cloud-get-issues-for-board-rest-api-does-not-return-the-correct-issues-for-boards-of-next-gen-projects/32951