Its a bug!
Do you have the bug #?
I realised that ive stumble upon same issue, but in a resource.
When one create a resource, and attached a resourcegroup to it.
Then you will realise you can never create a child resource below it.
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Then i found the problem.
The solution is to add this to the user group (administrator in your case):
+ resource group: [your resource group]
+ min role: [administrator]
+ access policy: [Resource]
+ context: mgr
But then again, why would this be working, but access policy of :[Administrator] not working?
Its weird, only effect create new document.
Hope it helps, cheers