This issue is caused by configuring permissions in Atlas CRM with groups that include capital letters. For example if you've restricted access to Atlas CRM to users of the group: "Test Atlas CRM" those users will not be able to see the Atlas CRM menu in Confluence.
We recommend that group names are written in lower case with hyphens which is also the best practice according to Atlassian. Instead of "Test Atlas CRM" you would have "test-atlas-crm". Not including capital letters in group names also avoids this issue.
We've reported our findings in the ticket https://jira.atlassian.com/browse/CONFCLOUD-67528
. We're resolving this incident because it is now tracked in the aforementioned ticket.