How to reproduce

• Go to cms
• Create a content A with user A in the page A where user A has right but bot user B
• log as user B
• Go to another page B where user B has rights and click on insert content : existing content, select content A

Actual behavior

• User B can edit the content A on the page B.

Expected behavior

• User B can not edit the content A

Why in another site you are using a "shared content" but not in the site on the original content ?