User Management FAQ says:

In existing instances, the 'users' group will continue to exist after the upgrade, but new users will no longer be added to that group by default. If you have given custom permissions to the 'users' group, we recommend migrating the permissions to these new groups.

Problem:

• Every new user is added to jira-users and confluence-users groups but not to 'users' group.
• All Project Role (Users) were set by default to the group 'users' in the past. (by Default, not manually, not custom)

Result: All new users can't browse or create issues in Jira because they don't belong to the 'users' group.
The way it used to work was changed and not all customers are aware of the change.

Solution:

• Add users to the 'users' group
• Add group 'users' to Jira Users Global Permission (that will trigger the user to be added automatically to the group upon creation)