In the Service Portal Page Designer, scoped Service Portal pages go blank when any editing is done.

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This is expected behavior because the editing is being done outside of the application scope.

In other words, if a page has been created in a scoped application, it is editable only when you are in that scope. If you are in some other scope say "global", you won't be able to edit it in either sp_page table or in the Page Designer.

Change the application scope to see the page properly.