According to this documentation, Space admins are able to revoke permissions (this behaviour works as expected)

https://confluence.atlassian.com/confcloud/permissions-and-restrictions-724765432.html

Page restrictions

Page restrictions work a little differently to global and space permissions. Pages are open to viewing or editing by default, but you can restrict either viewing or editing to certain users or groups if you need to.

Don't forget, every page in Confluence lives within a space, and space permissions allow the space admin to revoke permission to view content for the whole space. Even the ability to apply restrictions to pages is controlled by the 'restrict pages' space permission.

However, as expected from the documentation, space admins are not able to modify restrictions on a page (only remove).

The suggestion is for the space admin to be able to add themself to view restrictions of a page to gain access. Not being able to do this means the space admin has to make the page accessible by anyone in the space (by removing the restrictions), before they're able to view the page.