We are having this exact same issue, only on 4.1.12 and using lower-case. (Perhaps I should have created a new thread for this given that the original post is now two years old, but Hub described it so well I didn't see a reason to duplicate the effort ) The problem is only affecting a single address object, and in the FDQN logs it shows as "not resolved". Other similar entries are resolving correctly. What's odd is that this was working when originally configured some weeks ago. The firewall is able to ping the address by name, so name resolution is working. The problem entry is outlook.office365.com which is a mix of both IPv6 and IPv4 addresses.
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