A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear fima,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and I would like to convey that I have tested the behavior at my end with the Highlighted Content webpart with the filter condition as ContentClass equals to STS_Site, where the users who can access the SharePoint Online site with the highlighted content webpart can see all SharePoint Online site the user having access to.
However, to my knowledge, the behavior you are observing is not an expected and since you have tried to re-index the SharePoint Online site with no success, and I believe the problem should be further investigated by the related development team, where they can collect the HAR logs while reproducing the problem and perform the further investigation.
Note: I have checked the service health dashboard at the Microsoft end, where there is no related incident found.
So, I request you to connect with your IT admin of your organization (How do I find my Microsoft 365 admin? - Microsoft Support) and raise a new service request from the Microsoft 365 admin center so that the related development team will connect with you and investigate the issue further.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: We apologize for redirecting you to related development support team because the moderators in this community are limited with the resource for further investigation and to get a quick resolution to your problem, we requested it.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia