Thank you all for your replies, so it behaves like I was guessing, i.e. hitting the WAN address from inside the LAN. This makes complete sense for the behavior I'm observing and gives me peace to know that the ports aren't open to the internet anymore. It also simplifies some things for me as I can share some services to an isolated "work" VLAN which can look up public DNS records pointing to my WAN address and route them locally without having to set up anything more to make it work.
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