Some of the newly added elements (Confluence spaces) in the navigation are disappearing a few minutes after we've created them.
They also disappear for users who have enough rights to see the linked space.
You are experiencing the following bug:
Getting issue details...
When a Confluence administrator who cannot see all Confluence spaces edits the navigation and saves it, Confluence spaces the admin does not have access to disappear.
Progress on the issue will be documented on the linked bug ticket.