Hi desmong,
Thank you for querying in this forum.
From your description, it seems that you are trying removing users from permission group in SharePoint, however, you found there is an account named app@sharepoint appeared as administrator in SharePoint.
May I know if my understanding is right? If so, for the situation you encountered, we do understand the inconvenience caused and apologize for it.
And we have also tested it on our side and we cannot find such account after removing user from permission group.
However, per our search, the Account “app@Sharepoint” is related with SharePoint Applications, it is a system account. It belongs to the SharePoint Farms infrastructure. It was created to run on all the Site Collections that have Custom Apps and Workflows. For example, when we enable the Site Feature Workflows because we have Workflows running on Our Site Collection, these workflows will need to have permissions to run and they will use this account (i.e. app@Sharepoint) to run on the site collection. This is by design. All farms are configured in the same way and all with this account and all the site collections will use this account, if they need to. And this account will not affect the site collection functionality.
For the situation you encountered, we assumed that you may use a product that monitors SharePoint Online events and activities, for your reference: Office 365 Management Activity API schema | Microsoft Docs
You can also refer to this article: The app@sharepoint user in audit records
If the scenario above is not consistent with yours, you can also post back and point that. At the same time, please also provide the screenshot of “ap@sharepoint”. May I know if all sites have the same problem?
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Best Regards,
Sukie