I think the reason is you local DNS server which is used by NC and CODE server as well splitbraindns. using local DNS both systems know they live on the same server and connection happens “internally” showing localhost (127.0.0.1) as source address. consult Collabora integration guide for details. in this case I would recommend you adding 127.0.0.1 to the allowed list and you should be done 
there is no harm adding all localhost and private (rfc1918) addresses in the WOPI allow list - this addresses are not routable in the internet and can not be abused by and external attacker.
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