Experience Cloud: Roles are not created as expected when enabling the first external user on an account.

This behavior occurs when the "Minimize the number of roles created, which improves performance by cutting down processing loads" setting is enabled under Sharing Settings.

When this setting is enabled, roles are created as follows:

Enabling the first external user: 
A single role is created using the following format:

<Account Owner Alias> Customer Person Account
Note: The <Account Owner Alias> corresponds to the Alias field of the user set as the Owner of the account.

Enabling the second external user: 
Only when the second external user is enabled on the same account, the number of roles specified in Setup | Digital Experiences | Settings (e.g., <Account Name> Customer User) are created. 
Once these roles are created, all users (including the first external user) are reassigned to these new roles.

For more details on role creation behavior when the second external user is created (or when this setting is disabled), please refer to the following article:
Experience Cloud: Roles automatically created for accounts when enabling external users (customer or partner)

Additional Information:
The purpose of this feature is to improve org performance by preventing the creation of unnecessary role hierarchies for accounts that have only one external user on an Account.

If you wish to disable this behavior, navigate to Setup > Sharing Settings and deselect "Minimize the number of roles created, which improves performance by cutting down processing loads". 
Please note that once disabled, the full number of roles specified in your Digital Experience settings will be created for every account as soon as a single external user is enabled.