Case Team Members are Visible in Experience Cloud Sites when "Visible in Customer Portal" is Disabled

This is working as expected. The Case Teams feature contains a "Visible in Customer Portal" field that can be set to True or False. This refers to legacy customer portals on the Salesforce platform, and not Experience Cloud sites. User visibility in Experience Cloud sites is dictated by several settings, including, but not limited to:

Site User Visibility, located in Setup > Sharing Settings

See other members of this site, which turns user visibility on or off on a site-by-site basis

Sharing rules on the User object

If, for example, the see other members of this site setting is enabled in Experience Workspaces for your site, all site members will have read access to the user records of all other site members. Additional security features, such as Enhanced Personal Information Management and the Show Nicknames setting in Workspaces, can be leveraged to hide personally identifiable information on the records themselves. These security features will not change or remove the read access to the record when the UserRecordAccess object is queried, but they will help protect sensitive user data in certain fields.

If your business requires granular user sharing and visibility within the context of a single site, it is recommended to set the User object's external organization-wide default setting to Private and leverage existing tools like sharing rules, sharing sets, manual sharing, or Apex sharing. Please contact Salesforce Support if you find site user security features are not working as expected.