Authenticated users can't access a Confluence Instance opened to anonymous access

Issue Summary

It is possible to enable public access on Confluence Cloud. With the recent implementation of CONFCLOUD-30161, unlicensed authenticated users can access content that is opened to Anonymous access. For example:

• User has only access on Jira. 
• Confluence is opened to Anonymous access.
• User accesses Confluence while logged in. 

However, such scenario will only work in case the user belongs to the Instance userbase.
Below scenario won't work:

• User A has access on Jira from Instance A.
• Instance B has Confluence opened to anonymous users.
• User A (while authenticated to the Atlassian account) tries to access Instance B.

Steps to Reproduce

1. Create an Instance (Instance A), and add a user to it (User A).
2. Create another Instance (Instance B), and add Confluence to it.
3. Enable anonymous access on Confluence.
4. While logged in as User A (that doesn't have licensed access on Instance B), access Confluence from Instance B.

Expected Results

The user should be able to access the content, since it is publicly accessible.

Actual Results

The user can't access the content. 

Workaround

Accessing the Instance using the incognito mode or logging out of the Atlassian account would work.
Cleaning browser cache and cookies works momentaneously, until the user log into the Atlassian Account.